The first reason is we're not looking at eldritch blast vs sacred flame. We're looking at eldritch blast vs sacred flame plus another cantrip because sacred flame is free and eldritch blast is not.
Warlocks already get two cantrips by default and a third at level 4. You don't get this feature until level 6. If you can't find room for
eldritch blast within three cantrips, especially given the paucity of Warlock utility cantrips, that sounds to me like someone specifically trying
not to use it, rather than someone being fresh out of options. You also don't even get
sacred flame until level 3 when you pick your subclass, so that would mean spending the first two levels...apparently without any damage cantrips because those aren't free either?
The second reason is eldritch blast is an attack roll while sacred flame is a saving throw. Higher AC opponents with poor saves aren't uncommon and this impacts the reliability behind using which cantrip. It also means I'm not required to use my concentration on hex for additional damage and more open to concentration on something else.
Even if we give extremely lenient odds to the one and not the other, e.g. 80% failure on save vs. 50% success + 5% crit on EB, the average performance of EB is still going to be better. That's because any save-based cantrip is all or nothing, while EB can hit on any one of its two/three/four attacks. Even with those numbers, and scaled to appropriate values (e.g. +4 Cha mod),
sacred flame is all of 0.3 damage higher than EB+AB. The moment you get a +5 modifier, EB+AB is strictly superior.
I don't disagree that the way WotC has presented EB in such a way that other cantrips seem pointless for damage, but when radiant soul comes on the difference is 2d10+2*CHA+hex and an option to split the attacks among 2 opponents vs 2d8+2*CHA plus a different utility cantrip and a different use for concentration such as bane or the new blade ward instead of hex that's a decent enough trade-off and something that isn't a concern (IMO) until later.
Hex is mostly a trap at higher levels. You'll get much more bang for your buck from other uses, because
hex doesn't scale with level--or, rather, it does, but only for duration, not benefit. Also, you're incorrectly calculating the damage
sacred flame would do. It does not deal +Cha mod damage normally, Radiant Soul is what gives it +Cha damage at all. 2d8+Cha < 2d10+2xCha, and that's without accounting for critical hits, which
sacred flame cannot do. Unless you're already spending an invocation to boost it further...but I'm not sure that works with
sacred flame, because while you have it
prepared, I don't think the text says that it becomes a Warlock cantrip for you. It's just a cantrip you happen to know, so the 5.5e Agonizing Blast couldn't be applied to it. That is, Pact of the Tome includes the text "...and they function as Warlock spells for you," whereas Celestial Spells (like every other such feature)
does not say that the spells are, nor function as, Warlock spells for you. 5.5e Agonizing Blast explicitly requires that it be a Warlock cantrip.
It's when EB adds that 3rd attack that it becomes more questionable compared to some cantrips, and that 4th attack tends to win out over everything else.
The third attack is when it becomes unequivocally better. It's marginally better at level 8, and essentially equal at levels 6 and 7 (when the feature comes online but you still have +4 Cha mod.)
The evoker version only applies to cantrips. The celestial warlock also applies to hellish rebuke and wall of fire, and possibly more additions through feats or invocations. If I use that concentration for wall of fire I'm going to get the radiant soul damage on both the wall of fire and subsequent radiant flame spells.
Except you will only get the
wall of fire damage bonus one single time--hence why I said the feature is essentially worthless with that spell. If it were once
per turn, then it would pair extremely well with
wall of fire, but it is literally only once each time you cast a particular spell that deals fire or radiant damage. Hence why I say many things that
could be useful just aren't, because Warlocks specifically thrive on continuing/repeated damage and sustained effects. Hence, it's only cantrips that have any real relevance here; doubly so because the whole point of Celestial is that you should be spending some of your slots on healing, not damage.
Why? When I look at a free damage cantrip I look at with what I might replace eldritch blast, not "well this should just be eldritch blast with another name". It's not about investing in the same features. It's about some diversity in the options at that point.
There are only and exactly four utility cantrips in the Warlock spell list, in both 5.0 and 5.5e:
friends, mage hand, minor illusion, and
prestidigitation. That's it. In order to get to 6th level and have
sacred flame as your only offensive option, you have to have gone for levels 1 and 2 with no offense cantrips at all, and have to have chosen not to get
eldritch blast at level 4 when you pick up an additional cantrip. Given that
friends is essentially worthless in most situations unless you genuinely never expect to interact with or hear from the target ever again, that would mean you'd have taken all three pure-utility cantrips, rather than taking the one Warlock-specific cantrip that benefits immensely from it. At that point, you aren't just choosing utility; you are actively
avoiding doing damage, so we aren't even doing an apples to apples analysis anymore. It's a completely different playstyle at that point; predicating a damage analysis on "someone actively choosing to avoid strong damage options" is simply a different paradigm. It's not invalid in the least, but it's not appropriate to bring that up as a counterexample
when the topic is how much damage is being done. Which is what was being talked about.
When I'm looking at the celestial warlock it's not the damage that sacred flame can do with agonizing blast and radiant soul compared to eldritch blast and subsequent investment. It's the fact that the cantrip is a free damage cantrip and even if it's worse that eldritch blast that begs the question is it still enough damage to be worth other investments.
Okay. I am looking solely at the Radiant Soul feature, and saying it's a pretty garbage feature because it has almost anti-synergy with the Warlock's kit. Which is what I said from the beginning.
If a person doesn't think so then they'll probably skip celestial warlock. If a person does think so this is for them. If a person wants those benefits and also the damage benefits then my first thought is we can't always eat our cake and have it too. ;-)
My perspective, anyway.
Except I do think so--and still chose Celestial Warlock both because I vastly prefer its theme over other patrons, and because I knew the healing/support spells and Healing Light would be extremely useful to our group (as I'm the only spellcaster and the only proper healer.) I can think that this one specific feature is garbage, while still thinking that Celestial Warlock as a whole is fun, thematic, and useful. Much as, for example, someone could see the Battle Master as a fun and engaging subclass, but find the 5.0 version of Know Your Enemy to be worthless garbage. (The 5.5e one is much better--not a
great feature, mind, but it's now actually useful as opposed to being little more than a gatekeeper for pure roleplay.)