Um, 'scuse you, but Trump had enough of a majority in both the House and the Senate to pass Obamacare repeal with only Republican votes...and he failed. All. three. times.
Obama, on the other hand, had to work with an even thinner margin - especially after Scott Brown won in Massachusetts. Yes, the Dems had more votes...
but they had to overcome a filibuster several times, and barely did so, each time. The GOP under Trump didn't have to worry about a filibuster since they were working with reconciliation.
And even with the hundreds of proposals the GOP got added into the ACA, they still all voted against it...even though the Dems had allowed
many public meetings to discuss the ACA and how it would take form and eventually work. How's that different from the GOP's effort to repeal the ACA? The Republicans allowed very little discussion (and unlike with the ACA, did almost everything behind closed doors with no Dems allowed), and allowed no proposals by the Dems to be considered.
But perhaps the biggest difference is that with the ACA, Obama was front-and-center with the debate and discussions and knew most of the bill in-and-out...whereas Trump couldn't have cared less what happened, as long as the Obamacare he hated so much (if only because of the name) got repealed.