What parts can you rob from a spirit or acclaim that you cant rob for any 86-88 K? The body and interior stuff is all different. The engine stuff is all basically the same (with slight variations) on any K derivative except for the block it's self depending on year, never mind the fact that just about anything you need under the hood is available from a parts store anyway. Suspension stuff is available too. An 89 is the same to maintain as any other 86-88 k car.
In fact, looking at production numbers, the 89s are the lowest production out of the whole run of 81-89.
http://www.allpar.com/eek/k/build-stats.html Therefore, assuming a close to equal failure rate (Since 86-89 were basically the same car with very little changes) it's likely that there are less 89s than the 86-88s. The thing that makes the 81-84s harder to find is that they have carburetors which most idiots can't work on and they're harder to find parts for since they're older.
The same thing happened with Diplomats. Very few 89s were made and therefore you see less of them.