Because in the early 90s, they went to R134A, and when they did, the hose connection points got different heights instead of being the same. AKA stepped gasket surfaces. I guess because they were stupid and figured they'd make a "134" version of the same compressor. All the other internal parts are the same, I think. The machining for the hose connection is different. Thus, when you try to use an R12 hose assembly on an R4 compressor, you have a mismatch. Those idiots.
Non-stepped "normal" R12 connection points.
Stepped style. You can see the machining difference.
They try to use step washers, etc., to alleviate this issue, because most ALL of the remans and new R4 compressors you can buy are for 134. I have heard of blowouts using these washers, and it MAY be because they don't use the aluminum inserts that may or may not be included in the GM kits.
Note the washer thicknesses.
They have those washer kits you can buy...
A/C Manifold Seal Kit
ACDelco P/N 15-20058
GM P/N 2724887
These can be found with aluminum extenders and the yellow washer, and sometimes they don't have them. So be aware.
Ready Aire also sells a kit for R12 hose assemblys (non-stepped connector block) with the aluminum insert extenders and a yellow washer, something the GM kit doesn't always have. I wonder where GM gets them...hmmm.
Ready Aire P/N 2728
I think the best thing that CAN be done, is to find like a 1993 or 1994 Chevy truck hose (I think this is when they started using R4s with 134- there may be others as well) and get the connection side of the hose and have it welded on to your hose assembly. I know that's a PITA, but that will cut down on the fat/skinny gasket routine which usually ends up with a leak. ALWAYS look at the connection points when you get a new R4 compressor. Plenty people are unaware when they order a new compressor for a 1981 for example, and end up with a compressor that physically fits the mounts, but the hose connections are stepped. But that's the number that fits your 81!!! WTF? Apparently they no longer make the new compressors for R12.
Hope this helps.