> Theresa May took his place after some debate; she was vociferously pro-Brexit
I don't think that is accurate. See https://www.euronews.com/2018/12/12/what-is-theresa-may-s-vi..., https://www.bbc.com/news/uk-politics-38653681, https://en.wikipedia.org/wiki/Theresa_May#Brexit
Note that the number of Tory MPs which voted against her in the 2018 no confidence vote (which was an internal party thing) is almost exactly the same as the number of Tory MPs who voted against her deal. Even though she survived that no confidence vote it apparently presaged doom for this deal.
Former PM David Cameron put Leave/Remain to a vote, but publicly said he voted Remain. Resigned after the vote results. Theresa May took his place after some debate; she was vociferously pro-Brexit, and looked like the best option if Britain was to act tough when negotiating with the EU. A number of people dropped out of the succession race, gambling that the Brexit hassle wouldn't be worth it.
May survived a no-confidence vote in late 2018, but a number of members of her party have resigned because the deal on the table didn't sync with what's important to their constituents.
There are a number of factors, but May's insistence that net migration would fall to 5 figures was something people expected her to deliver. It's not going to happen.