For the Tory right, as far as I can tell, they just decided they had an ideological revulsion at any continuing involvement with the EU, so they couldn't bear the backstop. They certainly haven't been pragmatic about Europe, for the past 30 years.
It gave them what they wanted on immigration, as far as I could tell.
They seem to have a further list of what they hate about it, though:
It sounds like they think the EU will be so desperate to avoid No Deal that they think they can ask for everything they want and they'll get it, in a month. This just doesn't seem to match reality. I think they're incapable of seeing things from the EU point of view.
I was surprised that May's talks with Corbyn failed so quickly. I thought Corbyn, who is quite happy to leave the EU, and has supported a customs union, could have got something in to say "here was what I achieved" and they could have pushed it through. But his tactics have all been about how to become PM, I think; so he made sure May fell, and then didn't go for an immediate no-confidence vote (which would have allowed an election before the Oct 31st exit date), but waited until the only way to get Johnson out before then was for parliament to appoint a new PM, ie him.
And the Lib Dems looked bad when they'd said "we'll avoid no deal by any means possible" and when presented with this idea of Corbyn's, said "... except that".
All in all, the politicians really are a poor shower. The leak of the queen saying as much was quite right.