> why didn't they all vote for Obamacare and for the public option

In substantial quantities, they did. That's what Obama in the general election (and with slight variations, he, Clinton and Edwards in the Democratic primaries) campaigned on.

But they didn't get it, because by the time the health care bill got through Congress it had no public option, because while it was in the version passed by the House, it was killed in the Senate. Killed, that is, by a minority in the Senate -- it was the threat of filibuster, not losing an up-or-down vote, that did it in.