For me, the reason is the same one for my new machine being a Juki- all metal parts, you can clean and oil yourself, and if you care for them, they never die, unlike the newer computerized machines.
Featherweights, (Singer 221s) people like to take to classes. Most of the older machines are too heavy to lift easily. I am buying a 301, which is slightly bigger than a featherweight, but still portable. And my 15-91 will sew through absolutely anything without slowing down.
The old machines were designed for heavy use, and designed to last forever, unlike the newer machines. My Juki remains my go to machine, because I am spoiled by the thread cutter, but well made, easy to repair, and easy to operate are all worth having.