Maybe Facebook team are yet to fully understand bitcoin and hence they are dealing with altcoins for now, later on they can get on with bitcoin. Without bitcoin no other coin really make sense, except one wants to introduce scam to the public.
Mark Zuckerberg has billions of dollars and he has hired a team of the best professionals to help FB with Bitcoin is somebody does not understand it (although I doubt it too much). For sure, as money makes money, Facebook leaders have the only one aim - to make more on cryptocurrencies. I have almost no doubt that they will succeed here, and we will see the Libra project very soon.