Because they march according to the commands of their masters, not according to the needs of Americans. And their masters are the rich (who will benefit from removal of the extra tax they got from the ACA).
However, Heller is in a state that went for Hillary Clinton, and a state that stands to lose a lot under the AHCA if it passes. Furthermore, he’s one of only eight Republican Senators up for re-election next year, and bound to be heavily targeted by the Democrats in the midterm campaign. He can’t afford to give them any ammunition.
The other seven GOP Senators up for re-election in 2018:
Arizona — Jeff Flake
Mississippi — Roger Wicker
Nebraska — Deb Fischer
Tennessee — Bob Corker
Texas — Ted Cruz
Utah — Orrin Hatch
Wyoming — John Barrasso
Cruz, Hatch, and Barrasso were on the secret committee that put together the bill, so it’s doubtful any of those three would vote against it (plus they represent very red states). But the other four? If you live in any of their states, make sure you call and let them know how their vote will affect your vote.