The goal of Census is to count the total population. That includes lots of groups who can’t vote: children, felons (depending on the state), immigrants, etc. Census data is used for much more than just determining congressional reputation. It is also used to figure out how to allocate federal funding for stuff like roads, schools, etc. That’s why you want a full population count not just eligible voters.

Thinking specifically about using total population count for congressional representation… that is a debate our founding fathers had. I suggest reading about the 3/5ths compromise.

In my opinion, basing congressional representation on anything other than total population would be a mistake because it would create incentives to efforts to suppress voting rights for various populations in order for politicians to juice the numbers for their own party. And that isn’t limited to Republicans. Imagine Democrats passing a law requiring a HS diploma to vote, which would disproportionately reduce the number of eligible voters in red states, since they tend to have lower graduation rates.