Major hail damage while in storage, required extensive repairs to get airworthy, then was cannibalized for engines and parts at various times before it left Roswell. From there, it was due for a cabin reconfiguration and heavy check. A whole litany of delays, certification issues and other snafus arose so each time its service entry was going to slip past a summer season, United prioritized other airplanes and 121 sat idle. I am surprised it even came back into service, but we are finally here. From what I understand, it's one of the reasons the launch of seasonal EWR-KEF was pushed back this year.

Incidentally, after over five years in storage (wild) it is now probably United's "youngest" 752 in service, by the metrics an airplane's life is measured (hours and cycles).