@tim has given a good answer in terms of the specific vote. However it’s also worth looking at the reason why the Republicans couldn’t get behind a replacement.
The problem that the Republicans face is that the key elements of Obamacare were actually Republican ideas. This paper from the Heritage Foundation outlines the main points which were later introduced into Obamacare. When Mitt Romney was Governor of Massachusetts he introduced a statewide health law based on these same ideas. (Edit: added link to list of similarities and differences)
When President Obama proposed the Affordable Care Act the Republican party was faced with a difficult choice. They could have supported the ACA while pointing out that Obama had merely pinched their best ideas, but this would have meant handing Obama a major political victory and an important historical legacy. Since they hated Obama they couldn’t stomach that, so instead they decided to airbrush out their own history and position themselves in opposition. They managed to make hatred of Obamacare a major mobilising point for their base, but now they have the opportunity to replace Obamacare they are in a bind: they can’t create the scheme they had planned because it would be too obviously Obamacare with a red paint job, but they don’t have any new ideas to replace it.
Trump isn’t helping. He promised to replace the “broken” and “unconstitutional” Obamacare, but never got around to saying what he would replace it with. Then he discovered (to his immense surprise) that it’s complicated.
Former Speaker Boehner: Obamacare Repeal and Replace Won’t Happen
“In the 25 years I served in the United States Congress, Republicans never, ever, one time, agreed on what a healthcare proposal should look like. Not once,”
.. “They’ll fix Obamacare. I shouldn’t call it repeal and replace because that’s not what’s going to happen. They’re basically going to fix the flaws and put a more conservative box around it.”
Why Republicans failed to repeal Obamacare
How did Republicans fail to repeal and replace the Affordable Care Act?
They hated Obamacare but they never understood the Affordable Care Act.
It certainly (and this turned out to be very important) didn’t mean you had any ideas about what it did, how it worked or how many people were benefiting from it … or how to replace it.
— Obamacare created a new baseline.
— They don’t have a policy bench.
tend be much more skilled at campaigning .. than at the qualities that enable governing, like compromise and fact-based analysis.
— The one policy Republicans get is tax cuts, and little else motivates/interests them.
almost 50 percent of the cuts went to millionaire households. The 400 richest taxpayers, with average income above $300 million, would get a tax break averaging $7 million.
.. the extent of this Robin-Hood-in-reverse play was too much for moderate Republicans
— They’re ungovernable
John A. Boehner predicted what just happened a few weeks ago.
.. “In the 25 years I served in the United States Congress, Republicans never, ever, one time, agreed on what a health-care proposal should look like. Not once.”
.. People are learning his “art-of-the-deal” shtick, so they know that what he says Monday may well be totally different from where he is Tuesday.
.. Newt Gingrich, of all people, was compelled to tweet out the following: “Why would you schedule a vote on a bill that is at 17 percent approval? Have we forgotten everything Reagan taught us?”
Who Stopped the Republican Health Bill?
33 Republicans who would not budge from their decisions to vote “no” on the health care bill were key to causing its collapse. They can be divided into three broad categories:
15 Hard-liners
10 Moderates
8 Other Republicans