Like many other items, they became disposable when it got cheaper to buy a new one than fix the old one. Unfortunately this is happening with just about everything. Manufacturers have figured out they can charge ridiculous prices for spare parts and labor (local labor at least) is a lot so the guy that can't fix his kids bike is looking at 75 bucks for a minor repair and for that he can get another walmart special.

Same thing happening with appliances, electronics, and so many other things.