Well, I have the benefit of being a constituent.
(1) Zac’s result in 2015 was also an extraordinary swing as well as he benefitted from a backlash against the LibDems for the coalition. This has been replaced by a blacklash against Brexit. Richmond Park was a Lib Dem marginal in 2010.
(2) The LibDems had far more troops on the ground. We were inundated with leaflets from them, three a week at least, including a fake weekly newspaper, which hid its real purpose in very small print. They had a lot of activists on the ground, 1,000 were bussed in one weekend. By contrast, Zac had many Tory activists who remained loyal to him, but greatly missed the National apparatus that was available to the Lib Dems.
(3) He has been a very good local MP and is a decent bloke who has kept his word, an example more MPs should follow. He has certainly been by far the most visible MP representing me.