Because all races have to be identical.
I love how people(not singling you out here) are like, "Races can't have +2 strength or intelligence, because that's superiority over other races and bad!", while at the same time being like, "We can give them powerful build instead, so that that they are are stronger than other races and not superiority, and that's good!"
The self deception involved with being okay with one and not the other is astounding. Being stronger than other races is being stronger, no matter how you represent it. +2 to to intelligence is the same as more skills, which is the same as perfect memory, which is the same as...
You can't have racial abilities of any kind that other races do not have without having the "superiority" that people are against with stat bonuses. I put the superiority in quotes, because there is no such superiority in D&D. D&D gave elves +2 dex, dwarves +2 con, etc. which made all the races equal on those grounds. The same with the other racial abilities like the extra skills vs. fire breath vs. halfling luck, etc.