It's because the Brexit Party had the conservatives over a barrel. A good portion of their membership were leeching to the Brexit Party and they were facing a huge election loss.

For decades now, the left / centre left vote has been split across Labour, the Lib Dems and the Greens. The Tories are largely unchallenged for conservative voters. The Tories having to compete long term with another right wing party would be a death knell for them.

Standard disclosure that the US and UK left/right wing are very different, overton windows, etc. Even the right wing Tories would be considered reasonably socialist and left wing in US politics. Labour and the greens would be seen as radicals, like Bernie had a baby with Bernie and it was a super Bernie.