In addition to what’s been said above,Britain has a real thing for the underdog.
It’s almost the case that Froome is too good. So when ”plucky" Thomas comes along and wins against all the odds, everyone votes for him.
There is also the local factor. Everyone in Wales will vote for G same as everyone in Scotland, voting for Andy Murray. Everyone in Bury probably would have voted for Simon Yates. Froome doesn’t have a local and that counts against him as much as anything else.
Personally,I couldn’t care less about the sports personality and,I suspect,neither does Froome