Probably it was easier to adopt an already existing token instead of creating their own. I'm not 100% sure, but I think when they adopted FUN, it had already crashed deeply after its previous ATH.
FUN token reached its ATH (which was around $0.19) at January 2018 when many other shitcoins reached their ATH. Each FUN token was worth around $0.03 at February 01, 2021, when it became available for buying on freebitco and now each token is worth only $0.003.