The short answer is that old Mordekaiser was too toxic in terms of gameplay, where he was either cc'ed and died and basically amounted to nothing other than a walking damage dealer with no cc or mobility of his own (outside of his conditional W, and that was a relatively recent development in the champion's history) or he was a monster for whom he could handle multiple people facing him simultaneously.
He was much fun for those who remember old Mordekaiser, but he was oppressive to face for anyone who didn't pick a counter into him; like what the forums complain about with champions like Zed and Yasuo: really fun to play as, but horribly unfun to play against.