LMAO the Guardians became a legit pop culture phenomenon.
One movie already has a script, a director and is currently casting at least two of the characters, likely for an Endgame post-credit appearance. Given the far reaching plans, this was also likely plotted out at least a few years in advance.
The X-Men and FF, meanwhile, aren't even legally usable just yet (it's a given the deal will go through but it's still yet to be finalized), and outside of vague ideas probably haven't been heavily planned out yet. It's also looking like an increased likelihood that if those characters are in Phase 4, it'll be at the end given all the other movies we've currently had announced in the planning stages (we've got Spider-Man: Far from Home, The Eternals, Shang-Chi and Black Widow coming in Phase 4 plus likely sequels for Black Panther, Captain Marvel, Doctor Strange, the Guardians of the Galaxy, Ant-Man and the Wasp and the Avengers).
That's less "taking precedence" and more common sense. The Eternals are coming out far sooner than either the X-Men or FF, so they likely get the tease. It's not that shocking.