So, as someone with only vague comic memories, how often did a Smart!Juggernaut make the pages?
He's not, and never was an idiot. Of average intelligence, maybe. But not stupid, contrary to some film adaptations and how they portray Juggernaut. His hatred of Xavier stems from something stupid. Namely that Cain Marko's father was a gold digger, trying to get his hands on the Xavier fortune. And since Cain was a bully and thug even as a teen, dear ol Daddy would punish Cain for disrupting plans, eventually deciding to ship Cain off to military service to keep the boy from ruining said plans, not realizing it was the secret telepath in the household that actually was a threat to the plan.
Yes, the hatred for Charles Xavier is entirely because Cain Marko is a bully, and his dad would punish (then exiled) Cain for being a threat to an (doomed to failure) attempt to co-opt the Xavier fortune. And this has guided a lot of his actions. But the reason Cain is a criminal merc is because he's greedy, has no real skills other then fighting, and has no real morels. Even then, he's smart enough to know when to get while the getting is good and can strategize (although he doesn't often do so). Then again, he doesn't
need to strategize too often thanks to the powers granted to Cain.
After re-reading Juggernaut's 2nd appearance in the comics, and with knowledge that his strength and unstoppable nature were actively reduced when he tried being a hero who limits collateral damage... I'd guess that he had control of the aura stropped because Cain
didn't cause enough collateral damage when he had full control of it. The writers decided that the ability was dumb, so didn't keep it. But in-universe I'd guess Juggernaut was granted said control over the aura due to Cyterok being pleased with his actions initially, only to have said control stripped away due to displeasure in how Cain used it.