Part of the reason is they'd been building the same basic car since the 1976 Aspen and Volare, only with different trim.
The design has a few basic weaknesses.
First is the front suspension. The transverse torsion bar setup did not hold up well as the cars aged, but in a babied Fifth Avenue it may be OK. Otherwise, wear is often associated with chronic inside edge tire wear.
Next is the drivetrain. The plastic timing chain gear, lockup torque converter in the transmission and 7.25" rear axle are all weak spots. Timing chain is an easy fix, torque converter not so easy. The rear axle can be fixed with a swap to an 8.25" axle and some even had this better axle from the factory. The complex computerized ignition and feedback carb can have problems, but it's not really that difficult to make work right or swap out.
Last, the cars are not efficient. Lack of fuel injection and no overdrive in the transmission meant a 2.2 gear in the rear axle. The car lugs in town and does OK on the highway. But it's far form easy on gas for the size it is.
With reasonable care, they are pretty durable though I'm sure many are junked for transmission or reaer axle issues if they are beat on.