Because most use it wrong.
In modern low encounter/day 5e, you open a fight with Lightning Arrow or Flame Arrows then cast Hunter's mark turn 2. This gives you a strong burst that last from level 9 to the mid teens.
No.
First of all, you claim people "use it wrong" then just claim they should use it at the beginning of the fight. Which is... just using it. There is nothing special about the first turn of combat that makes Lightning Arrow more effective.
Secondly, the reason it sucks is quite simply the math. This is a 3rd level spell with a limited AOE. Since we are talking rangers, this means we are 9th level minimum. The AOE does 2d8 or an average of 9 damage, save for half and dex saves are fairly common to pass so at least half the targets are likely to take... 4 damage. FOUR damage. From a 3rd level spell. This is WORSE than a 1st level spell like Burning Hands, which will on average with the same considerations deal 5 damage.
Additionally, they only take 4d8 (18) damage on a hit. Which, yes, is more than you would likely do on a normal hit (2d8+mod ~ 3d8) but this is a THIRD LEVEL SPELL. This is your most powerful attack. The average expected damage for an AOE 3rd level spell is 6d6 or 21 damage, 10 on a save. For a single target it shuld be 5d10 or 27.5.
Like, you can claim it exists, but it is so terrible it really shouldn't. It feels bad to use it.
This allows a ranger to convert more of their spell slots to damage.
That's how you ran a ranger mid level 3e and 5e. Some in 2e depending on your magic rules. Buff. Buff. Blend. 4e skipped the Buff.
However 5e forgot most of the ranger melee buff spells. Paladin theirs plus more.
I don't need more ways to convert spell slots to damage. You seem to be arguing an entirely irrelevant point. The existence of Flame Arrows (which sucks as well, since it is concentration and is just... hunter's mark but for a higher level slot), Lightning Arrow, or Conjure Barrage (which is ALSO too weak for its level) does not give the Ranger Ranged smites.
And even if it did, even if Lightning Arrow was an incredibly good ranged smite spell... it still works differently than the Paladin smites, so it would be fine.
So
Fighter: Strong Attack forever
Ranger: Buff Buff Blend
Paladin: SMITE
Ranger and Paladin don't have the slots to cast 2 spells every combat plus a few out of combat casts in a 6-8 encounter day. So they work best at bursting. And WOTC make the Paladin better. The Paladin has a built-in slot-to-damage feature, heavy armor, and a nonspell healing.
Except the ACTUAL design of the 5e Ranger SUCKS at bursting. They have essentially no good damage spells after 1st level, and they seem to be designed to encourage a long-term, ongoing buff to their damage.
You keep making the claim, but it seems entirely unrelated to the reality of the Ranger spell list.
Which is the issue with Ranged Smite.
The only thing a ranger has going is ranged burst. A ranger doesn't get 8 slots until level 9. Meaning that the fighter has a better consistent attack in a long day and paladin burst melee better in long or short days. Rangers just have burst ranged attacks.
No, the ranger actually is terrible at ranged burst damage. It doesn't matter that they don't get 8 spell slots til level 9, because Hunter's Mark lasts for multiple encounters, especially for the ranged ranger who won't often get hit. And even WITHOUT spell slots, the Ranger is likely far out doing the Fighter, unless they have a subclass that gives damage buffs.
And fighter subclass damage buffs.... are a limited resource.
I'm not trying to be rude here, but you seem to have a fundamental misunderstanding of the class design of the 5e Ranger, Fighter and Paladin that is causing you to make these statements. The actual best two burst damage characters are the Fighter and the Paladin, while the ranger is actually designed to do consistent damage over time. Your entire paradigm is just wrong.
Again, with the caveat that everything changes at 11th level where the paladin and Fighter get consistent damage upgrades, and the Ranger gets (essentially) nothing.
