But why aren't 85mm lenses as easy and cheap to make? I would love to have the Minolta MD 85mm/2, but the speed of the lens seems to keep the price up on eBay. Why wouldn't Minolta have made an 85/2.8 at a more economical price point? As regards Minolta MC and MD lenses, the original poster is definitely right: 135s are plentiful and cheap.
Short answer: go ask minolta ~why~
Sorry to be snippy, but your question sounds more like a complaint.
Long answer:
Regarding manufacturing: I think that at the time, consumers were more interested in a telephoto lens to get close to the subject. 135mm seems to be the longest focal length available in a compact size lens. I dont think too many people would want a 200 or 300mm metal barreled lens around their neck. Also remember that fast films were not as good as they are now, so shooting with a long lens may have required a tripod. Finally, note that most 135mm lenses share the same filter thread as their shorter cousins.
Regarding current trends: ask around for advice on a focal length for portraits and the majority of answers will probably say 85mm or 100/105mm. Not many will reccomend a 135mm for anything.
Personally I have a Nikon Series E 135mm f2.8 and I like the focal length very much for walking around with. It is reletively compact yet bright, and gets you closer to the action than a 50mm. I've used it at a local nature preserve where I can get pretty close to the animals and at the zoo.
That said, I think a good tele zoom like a 70-210mm gives you better options for most situations than a 135mm. The 135mm is too long for family photo type shots where a perosn will usually be 5-10 feet from you.