Why? Did you try both and what did you think?
There were two reasons for my change, the Alacritty devs are really obsessed with speed, which is good, but.. means less features, even _optional_ features like ligatures. I like ligatures, and because they slow down the rendering slightly the alacritty devs do not want to include it[0]
The other reason is that I wanted a way to have my terminal have two different colorschemes, light for the day and dark for the night. It seems that it might be supported now with alacritty[1]? But it does also seem like you need an external script still to do it..
I went with WezTerm because it supports both natively, everything else feels the same TBH, but the fact that WezTerm has more features that are optional, is what I liked.
What I can say is, that I find wezterm configuration quite pleasant. Overall I think they are in the same league but Alacrity gets a lot more attention and that's why I like to remind people that there is wezterm too.