https://wccftech.com/bioware-explains-why-they-didnt-use-unreal-engine-4-in-mass-effect-legendary-edition/

Mass Effect Legendary Edition will launch on May 14th, as confirmed today by Electronic Arts. We also learned that this will not be a full remake, and BioWare consciously opted not to upgrade to Unreal Engine 4 technology, as that would have required remaking entire systems (such as dialogue trees).

Speaking to IGN, Director Mac Walters explained that going with the full remake route with Mass Effect Legendary Edition could have lost some of the magic in the original.