Because it's dumb that you gain more for being a tiefling celestial warlock than an aasimar one!
More in the sense of counting raw powers on a list maybe, like people did on Day 1 of the PHB 2014 with the Monk. Remember way back then people claiming Monk was overpowered because they counted their powers and saw the quantity outnumbered others and assumed that meant it's power was highest?
But more in terms of actual in-game power? I don't think tiefling gains more than aasimar from going celestial warlock. Indeed, Tiefling's attack cantrips are not very compatible with any version of Warlock.
Let's break it down:
Aasimar powers:
Darkvision: Decent (both have this so not relevant)
Resistance to Necrotic Damage: Decent. Necrotic is a more common damage PCs take
Resistance to Radiant Damage: Weak. Radiant is a very uncommon damage PCs take.
Healing Prof Bonus x d4 to any target 1/day: Decent. Used every day. In the Celestial theme of healing.
Light Cantrip: Weak. almost entirely flavor as someone always has light if needed.
Transformation Damage 1 min: Decent. Not huge but will be used every day.
Flight (or): Very good, will be used every day
Damaging Aura (or): Very good, will be used every day
Fear Aura: Very good, will be used every day.
Aasimar will use their Healing, transformation damage, and Flight/Aura every day with the Celestial Warlock. That's their core abilities and they're complimentary. Necromantic damage will come up sometimes.
Tiefling Powers:
Darkvision: Decent (both have this so not relevant)
Thaumaturgy Cantrip: Weak, entirely flavor
Resistance to Poison and Poison Spray cantrip (or): Decent, poison spray is weaker than Eldritch Blast
Resistance to Necrotic and Chill Touch cantrip (or): Decent, chill touch is weaker than Eldritch Blast
Resistance to Fire and Fire Bolt cantrip: Decent, Fire Bolt weaker than Eldritch Blast but benefits from Radiant Soul.
Spells:
Ray of Sickness and Hold Person (or): OKish, neither is a particularly good spell
False Life and Ray of Enfeeblement: Decent, false life will be used every day but Ray is weak
Hellish Rebuke and Darkness: Good. Hellish Rebuke will be used every day.
Tiefling will use False Life or Hellish Rebuke every day. Resistance to something will come up sometimes. However, most of their other core powers are attack cantrips which are objectively weaker choices than the Warlock's Eldritch Blast,
In terms of compatibility with the Celestial Warlock, Aasimar is better than Tiefling primarily because the Tieflings core attack cantrip ability is redundant or weaker than the Warlock's core Eldritch Blast cantrip (I think Fire Bolt benefitting from Radiant Soul catches it up to at least redundant and not weaker), leaving them with just situational resistances (and the Celestial even has one of those resistances) and two spells that are pretty good but not better than flight or the two auras.
Just because Aasimar loses two weaker abilities (light cantrip and radiant resistance) doesn't mean Tiefling is better for a Celestial Warlock. The abilities they use every day, or which are good when the situation comes up, are better for a Celestial Warlock than the Tiefling's abilities which are used every day or which are good when the situation comes up.
I do think an Infernal Tiefling would make for a pretty decent Celestial Warlock, with Firebolt, Hellish Rebuke and resistance to fire. They could probably skip Eldritch Blast and use Firebolt as their primary attack cantrip due to it's synergy with Radiant Soul. It's not as good as a Warlocks Eldritch Blast with those various combos and multiple bolts we all know about, but it allows for more flexibility in some choices. I don't think it's quite equal to an Aasimar Celestial Warlock, but it certainly competes with it and will be better in some situations due to enhanced flexibility. Both would be fun to play and have interesting flavor.