One thing to keep in mind is that UE comes at a hefty cost, from their official page :
a 5% royalty only kicks in when your title earns over $1 million USD
5% is a crazy amount of money on such numbers. If possible, it absolutely makes sense to reuse the engine that you already made profit with and you already have developers familiar with. Especially when you can have some paid partnerships; like they probably did with Nvidia to integrate their cutting edge features.
