Why would a pilot's armrest be a no-go item?
Perhaps it was just an oversight on the part of the creators of the minimum equipment list.

As I understand it, minimum equipment lists generally provide a list of those items which are permitted be inoperable. So if any piece of equipment is inoperable, and it's not on the list, that automatically means you're not allowed to fly.

(Going by the name, you'd probably expect a minimum equipment list to be, you know, a list of the minimum required equipment. But for one reason or another, it's not.)

So, in all likelihood, nobody thought to add armrests to the minimum equipment list. As a result, flying with an inoperable armrest was automatically prohibited.

However, things have since been adjusted: the FAA has added the armrest to the master minimum equipment list for the Boeing 757. So, it is now legal to fly your 757 with a stuck armrest, as long as it is stuck in the "up" position.