Because they don't have a challenge. They're not meant to be the JLA or the Avengers or second to the TOAA for that matter.
They're people with powers in a world that fears and hates them. And can KILL them. There's no sense of conflict now. Sabretooth? Phoenix. Apoc? Iceman. Sentinels? Polaris.
Mutants should be powerful but not JLA-powerful. It's not fun to read Psylocke flying around with the power to destroy mountains. We want to see Psylocke kicking ass as ninja or wearing armour with the butterfly effect. NOT fighting cosmic battles with a reality-warper....
The suspense is gone. The feeling of tension is gone. Back then, we saw the X-Men struggling against their enemies and winning through sheer teamwork and leadership. They're supposed to be a team. Not a group of heralds. It takes away from what it is to be a mutant: a freak with extra ordinary powers. Not a freak with the power to rewrite creation. That's no fun.
I want the feeling that the X-Men really are struggling against humanity AND that Sentinels are an actual threat AND that their enemies aren't mere hinderances to their awesome might.
Which is why Uncanny X-Men is so unreadable while Astonishing X-Men is. UXM has the friggin Phoenix-junior on it. AXM has several powerful members but they can still lose. We want to see them lose sometimes..cause when they win it'll be much sweeter.
The X-Men aren't the JLA. The JLA were written to be an unstoppable beacon of justice. The X-Men are a ragtag group of muties just trying to survive and get through the next day....NOT blowing up continents with a plasma beam....
🙄