ehhhhhhhhhh i dunno. im no competitive expert or anything but to the best of my recollection, shuckle has always been kind of a punching bag, and not in a wobbuffet sort of way where it can counter. having eviolite would certainly make its defenses more impressive, but that wouldn't really do a whole lot to mitigate its pretty weak typing. i imagine steel isn't a super common offensive type out there, but rock and water are and there's only so much of a beating shuckle can take. not to mention that a lack of reliable recovery aside from rest hurts it a lot; you're giving your opponent two free turns to set up which isn't a good thing.
ultimately i think shuckle is kind of one of those pokemon that's just doing a great job being good on paper defensively than in practice. someone else is free to correct me on these points though.