Essentially the vote of two US citizens does not count equally if you count everyone.
If one lives in an area with more illegal immigrants, that US citizens votes foe him/her but also for the non voters in the area. This is not a ‘one person one vote’ situation and it’s a real concern if you believe in equal representation.
Ideally you’d have a citizenship question and you count only citizens for electoral college allocation and everyone for non electoral college matters. But that creates concerns of undercounting etc.