Because I think frostbite is simply the best cantrip with a save. Forcing disadvantage is IMO worth more than the additional damage from toll (plus too many creatures are immune/resistant to necro damage).
Thunderclap is good for a swordmage or EK who likes to get in the middle of enemies. I don't think an evoker should be aiming for that.
Sacred flame is not worth multiclassing or taking a feat for. (Also, it doesn't let you attack creatures with total cover, it just denies the save bonus from half or 3/4 cover.)
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I'll admit that part of the appeal of frostbite is that it is actually evocation. I don't really get giving evokers benefits for necromancy or conjuration spells.
Thunderclap and Sacred Fame are also evocation and disadvantage from frostbit is lost per the ability we are talking about so its irrelevant to the topic unless you need 1/2 cold damage for some reason (you likely don't). If your saying your ignoring the ability and taking your favorite cantrip... sure take it too, its not like you only get one cantrip so taking thunderclap and sacred flame gives you the option when its relevant and will be far more effective at that time. That's the point of taking multiple combat can trips. Cold damage has a lot of immunity and resistance too but I will agree that GMs tend to lean on undead as unsympathetic evil which tends to make necrotic resistance more common. Truth be told Toll the dead is the most damaging single target spell with a range over 10ft and poison spray is not only shorter range but poison resistance and immunity is actually more common than necrotic. If your stuck on the secondary effect of frostbite that's fine...But that's also a bit off topic because your taking with disregard to the ability being discussed instead of including it. (Unless your agreement it that Potent Cantrip sucks because its not useful with "the best" evocation spell that qualifies.)
I would argue that if you want a single target EVOCATION spell that it not as likely to be resisted and is good with Potent Cantrip...
Lighting Lure d8 with 15ft range is actually better since it does half damage on scaling d8 lighting damage and pulling an enemy 10ft toward you from 15ft away then moving 30ft away means you can free an ally to move out of combat range and move out of combat range yourself which so a melee opponent has zero chance of hitting either of you. It also uses strength save instead of constitution save which will make it far more effective because dex fighters/rogues are likely to dump strength but NO ONE wants to dump constitution so potent can trip is going to trigger less but be more effective when it does. (I sill prefer Toll the dead to stand further back and do damage but if you want evocation...)
There is no need to mulit-class to take Sacred Flame and taking the feat Magic initiate (Cleric) and
you don't just get sacred flame. I would likely take guidance and healing word as my 1st level spell. Healing word is also (evocation) and its a good once a day save a party member a wizard might normally not have and for story I would be a minor devotee of Ioun who favors knowledge and would support wizards who seek knowledge for the aid of others.
Also, Jeremy Crawford comments Sacred Flame here
http://media.wizards.com/2017/podcasts/dnd/DnDPodcast_01_19_2017.mp3 @ 36:20 after explaining line of sight and invisible wall interactions he specifically states that Scared Flame can attack players in full cover HOWEVER you have to see them so you can hit a target behind wall of force, windows / invisible creatures, or any other barrier that would stop spells traveling from the caster to a target but you can see through. If there is a little crack and you see the target but its still considered enough to be considered in full cover you CAN target it and hit it with sacred flame without any penalty where no other spell can.
