Republicans are currently making it harder for left-leaning populations to vote, by closing polling stations in urban areas or opposing vote-by-mail. Automatic voter registration is being actively resisted. That is “why”.

Voter registration became a thing in the 1800s to limit the voice of immigrants, adopted state by state.

Oregon first, and about a dozen other states since, have made registration automatic when you get a driver’s license or state ID.

So yeah it’s pretty much always been like this.