Promising to repeal Obamacare was a tool the politicians used to manipulate Republican voters, much like promising that Mexico is going to pay to build a border wall for us.
It lets the politician promise something they can't deliver, perform a token effort at delivering at the promise, and keep you voting for their party in hopes that one day they will deliver on that promise. This worked well during the Obama years. There were a ton of failed votes to repeal Obamacare that had no chance of success. The politicians knew it had no chance of success, but it let them claim they were trying.
Of course, this is only sustainable if they can't actually deliver on the promise. Now that the Republican party is in a position where they could repeal Obamacare, they're in a bit of a predicament.
If they successfully repeal Obamacare, then they lose the tool that they relied on to stay in office and risk being voted out of office by the people that suffer from their vote.
If they vote against the repeal, then they risk their constituents realizing that they never actually intended to repeal Obamacare - that they were lying to get votes.