I think some Rugers use MIM small parts as well. The problem with 1911's is MIM is used to replace parts that were designed to be forged parts. If something is designed to be forged and you start making it out of a lesser material... not the best idea. I don't know if Smith has had any problems with their MIM, but I believe Ruger gets away with it because they designed the part to be MIM from the get go so its sufficiently strong.
The problem with 1911's is also the QUALITY of the part. It seems like good MIM parts might be ok for use in these pistols (I've never heard of Wilson having problems), but then you look at DW's, who are breaking MIM parts left and right. DW's obviously have MIM parts of much lower quality than Wilson parts. There are probably MIM parts out there that could outlast some of the barstock parts, its all about quality. And its much easier to get a bad batch of castings than it is to get a bad batch of billets or forgings.
MIM "should" be better than cast parts, but lots of the cast parts such as Caspian or Brown slidestops are made a lot better with tighter quality control than a lot of MIM parts. And most cast parts are machine finished so they don't have that cheap "just came out of the mold" look.
And 1911's appeal to different people than say a Ruger pistol or Glock would. Lots of 1911 owners care about the appearance and apparent quality of their gun, not just whether it goes bang. They want to have the best. MIM ain't the best. And 1911 owners have a ton of different brands to compare to. Most pistols can't be compared to any other gun in the same way a Springfield or Kimber can be held next to an Ed Brown and WWII Colt and compared.