Whether required or not, D&D 5E is the largest selling RPG in the world. It accounts for literally millions of players. And about 75% of those won't even think about a different game engine, for a variety of reasons.
For a small company, basing off anything else is going to merely cut the potential sales by more than half.
Stargate, likewise, isn't the biggest IP around.
FFG got away with Star Wars by several factors: FFG is a long standing powerhouse in the industry; Star Wars is one of the highest grossing IP's ever, and FFG went all out on the public beta. Star Trek RPG