If you know anything about the Commodore 64 (and other Commodores, really) you know that Commodore really went the cheap route and produced some awful power supplies. The Commodore 128 PSU isn't quite as bad as the C64 version, but it's nowhere near perfect.
If you open up the PSU for a C64, you will see that the insides of the brick are completely filled with epoxy. There is no way to salvage the parts or repair them.
My assumption is that Commodore didn't want average folks repairing their own PSU when they went south (which happens a lot). And, Commodore would much prefer you buy a new PSU. But was that really the reason? Or, did Commodore think they were making them safer by doing that?