Wow. If it's not single seat, single engine and capable of accelerating in the pure vertical, it's just an income generator to me. My income and the company who owns it.
Back to why people stay on the 75.... Hometown flying is a big one. I've flown with several guys who sleep at home every night. They are senior guys with their high five tucked away, all the international adventures filed away, and now they stay close to home to maintain their health and work on hobbies. There isn't an easier job at Fedex than flying day lines in and out of your hometown airport. Conversely, there isn't a worse job than some of the junior 75 lines, bouncing through towns at night that shouldn't even have airports. The senior side of that explains why some guys stay on the 75 when they could easily go somewhere else. And the junior side of that explains why some guys would rather hang out in a widebody right seat than move to the left of the 75. I've been extremely junior on the 11 and very senior on the 75. I like flying around the world but given those two options I'd take the senior life on the 75 in a heartbeat. It's the healthiest I've ever been flying at Fedex.