Heh... I've kind of felt that it's not because of removing half-elves and half-orcs as specific species write-ups would be the reason to remove tieflings, aasimar and genasi... but rather the existence of the Sorcerer class that should do it.
Almost every Sorcerer subclass is about having some magical being in your family's ancestry or someone way back when being infused with magic from some source. Which is pretty much how tieflings, aasimar, and genasi come about. So why those three are species and yet divine souls, clockwork souls, storm sorcerers, and those with draconic blood only get to be one specific class is kind of meh to me. The tiefling, aasimar, and genasi "Planetouched" should by rights be Sorcerer subclasses too and not species at all.
Quite frankly, this is one of the main reasons why I don't like the Sorcerer class. It's wishy-washy to me and most of its subclasses should probably have just been species to begin with.* The only subclass I like for it is the Wild Mage, because if the Sorcerer is the only spellcasting class that gets to avoid having a specific flavor or story and can be just the blank-slate "I have magic!" option for players... I think being unable to completely control said magic is the price you should pay to get that openness. Having a generic and flavorless spellcaster class can be fine (and I agree with those others who have said the Sorcerer should probably get the same option to select which spell list they want to use like they gave to the Bard)... but you need a slight bad to go with the good in my opinion. Plus then remove the 'Planetouched' as a species option and just make them Sorcerer subclasses.
*Unpopular opinion that I have no illusions will be completely ignored by any powers that be.