Ignorance is my guess, though from what I hear they aren't uniformly well made. The air filter cover is held on with four long screws, making the filter such a pain to replace that I would guess it hardly ever gets done. HD probably sold tens of thousands of them, they have some weaknesses, not the least of which is the target market, and at the first sign of trouble the saw shop will say it's not worth fixing.
I know I've told this story before, but what the hey. I bought my 2150 in 1998, and it was my only saw for a long time, cutting up everything that blew down around my place and the neighbors' places. I was mostly a recreational woodburner, maybe burned a cord a year and gave the rest away. Then it did about three cords a year when I moved into a house with a wood stove for a year, and then it wouldn't oil. I kept using it with an oilcan until it wouldn't idle. When I took it to a saw shop they asked how long I'd used it. I said seven, eight years, they said it's junk, pitch it, it's got no more compression, and the oiler is a difficult fix that will cost more than the saw is worth. I put it on a shelf -- have a hard time throwing anything with an engine away, you never know what you can make from parts of it.
Well, I got more saws, discovered this site, and decided to play with the "dead" 2150. I discovered that the oiler cost $14 and popped right out. I also discovered that the idle problem was caused by a seized clutch bearing, put another one in with a new spur sprocket. New 30LP chain from Bailey's, and the saw starts in four pulls and cuts a lot better than most would expect. It's the saw that sits by the back door and it gets the oldest mix and all the jobs that are too dirty or insignificant for a "good" saw. But if I'd listened to the guy at the saw shop it would be in the junk pile with the rest of them.
Jack