The problem, according to some, is that being forced to pick a set of traits from one parent and leaving the influence from the other parent purely aesthetic, echoes a very common way in which mixed-race people are often marginalized, being treated like they can’t be both, they have to take after one or the other.
Personally (and for what it’s worth, I am a mixed race person myself, but look white as the driven snow), I can see this criticism, and I agree that the “pick one or the other for your stats” approach isn’t ideal for representing mixed heritage, I see it as less of an issue than treating some mixes as more valid than others by giving them complete species entries but not others.
The ideal solution would of course be some kind of build-your-own-mixed-species system, wherein you could pick a trait or two from each side. But since 5e’s species design isn’t uniform enough for that to work easily, I can see why WotC hasn’t decided to go with it, as far as we know.
My recommendation to anyone who wants to play a mixed-species character and doesn’t want to just pick the stats from one parent, would be to use the custom lineage tools from Tasha’s. It’s not a perfect solution either, but there really is no perfect solution within the limitations of 5e D&D’s rules structure. It does seem from my perspective to be the “least worst” option available.