Politics, as
@Python1287 said.
The F-35B is the only version that can hover, the Marines are the only ones who have that version, and that's the only version the Marines have.
The
Marine Corps always seems to be subconsciously worried that their funding will get cut and they'll be absorbed into the other services. The Army has successfully done amphibious ops (more like used to but don't anymore- it's been decades), the Navy flies aircraft and helicopters off ships, the Air Force flies off land, the Army has plenty of doctrine—and real capability when there's a crisis in the world—for quick reaction force/light/agile/elite and they've flown their helicopter off of Navy warships from time to time. Those things are kind of a political threat because politicians might look at them and ask what the Marine Corps brings to the table that's so unique.
It's not an entirely fair question but there you go. The phrase "second land army" has come up a lot in the last twenty years of campaigns in Iraq and Afghanistan- food for thought about politics.
(Your question about why the hard stance agains the Super is a fair question. The political question is mixed.)
