Obviously history plays a part there but V8 integration was not particularly better and mostly the high number of Nix users contributors over the years made it hard to use things made by GAFAMs.

Looking at the code, spidermonkey was added 2009, mayber earlier (stopped looking at around 2009). V8 was released in 2008, so the decision probably made sense back then, considering V8 wasn't nearly as big as it is today.