The US government doesn't work like that. There isn't some logical person making logical decisions.

For this change to occur, many senators and representatives would have to be directly and personally incentivized to make the change.

They aren't.

Moreover, they're incentivized to do the opposite. Expats aren't a sympathetic group to any major voting demographic.

And how about they make this service opt-in. I was born in the US but I have had nothing to do with the place for 95% of my life and I don't want their help. Why must I fill out their paperwork every year or pay them money to opt-out (which I can't even do anymore because of the current backlog).