A sorcerer with a red dragon bloodline is going to want to cast nothing but fire spells.
Sure he might. But he probably still won't take Elemental Adept. +2 Cha means +1 on attack, +1 on Cha saves, -1 on monster saves vs. his spells, +1 on some skills, and +1 damage on a 12D6 attack. Compared to +2 damage on the same 12D6 attack and the once in a blue moon extra damage vs. a resist monster.
Where exactly does this specific feat trump the general +2 to ability score?
And what about the sorcerer with the copper dragon bloodline? There are probably very few monsters with acid resistance compared to fire resistance. Is this feat anywhere near as useful for this PC (not that it is really useful for any PC)?
If one monster in 20 has resistance that this overcomes (and the monster would not have died or run away anyway or even been targeted with the given spell, even if did have resist), that means that this feat has some small utility about once every three levels compared to +2 Cha that helps out 5% of the time for a wide variety of circumstances maybe several dozen times per level.
If we are talking a feat that won't even be considered until level 12, that's a feat that helps out 3 to 5 times by the time the PC gets to level 20. Whoop de doo. Sure, those 3 to 5 encounters it helped waste those resist monsters, but out of well over a hundred total encounters from level 1 to 20, who is going to remember that?
Not even in the same ballpark as other feats.