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

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.