They were bought by a much larger C++-Shop (Sony) and were trying to get the benefits from a larger eco-system. In the end they were bought for their talent, their experience, their brand - but not for their technology.
For Naughty Dog it could also have been the right moment, since from a certain point in time the game platforms are getting so complex that making custom inhouse game runtimes may no longer make sense for smaller game studios.
Reddit OTOH had never delivered anything commercially successful in Lisp, little experience with Lisp, but heard that it could be cool. They used unproven tech. Naughty Dog used proven tech and had enough experience to do low-level systems programming for a brand new game platform. Which is really an outstanding achievement. Reddit had only a rough prototype in Lisp, Reddit then switched inhouse to other technology.
The early Reddit in Lisp: https://github.com/reddit-archive/reddit1.0
I’m also fairly certain that rewriting in lisp would have been nearly as successful: the benefit came more from learning about the problem space than from switching languages.