Having two big esport games under their belt is a pretty big deal, as well allows them to consolidate those teams under the same company. Psyonix has a lot of experience with developing an esport and Epic Games might also be looking for help developing Fortnite as an esport.

I think it makes good sense. A lot of the things they used to scale Fortnite to the behemoth it is today could also be used for Rocket League. Fortnite's frequent content updates, game events and their business model, could help Rocket League achieve some of the same success (not that is not already big but Fortnite is another level). They already use unreal so another opportunity to leverage existing talent. Lastly the 250 million Fortnite players might also be interested in Rocket League if it was pushed a little.

Rocket League is mostly about its potential as an esport. Buying everything seems like the straight forward way to handle this.