Easy, because right after it was passed and helping millions of Americans, Republicans won both the house and the senate.
Tell me how Obamacare helped millions of Americans by screwing so many other millions of Americans so badly that they immediately voted the Democrats out of control of the entire Congress for passing it?????
Obamacare failed for two simple reasons.
1) It focused on the number of people covered, not on costs
2) It was never fully implemented.
With respect to #1, Republicans blocked attempts to control costs, though Dems should have made that more of a priority.
With respect to #2, Republicans blocked attempts to get everyone signed up. Insurance only works when the people that don't need it today (the young) are paying into the system so that later when they do need it, there will be money in it for them.
As far as people voting for Trump exclusively on this matter. That's laughable. If people universally hate the law so much, why is it's popularity so high? Why are Republicans having such a hard time repealing it? Because they answer to their constituents, mostly in states that voted for Trump.