Three reasons: 1. Technology. 2. Process. 3. Results.
1. An older camera has a mantra. It has soul, it has history. It also forces more manual intervention - some of my main cameras are totally manual, with no light meter. I call that the salt n' shake value. The more that you put into something, the more valuable the results feel to you. I was disillusioned with my DSLRs. All auto this, that, and another, with burst mode. Just no fun. I don't want robots to take over yet. In addition - price. I can buy a cracking compact camera for 50p. Okay, if I want better, I can buy an ex studio medium format system camera for less than two hundred quid.
2. Process. I like developing and scanning (I'm a hybrid film photographer) my own b/w film. I like the smell of fixer, and I still get a kick out of un-spooling a wet slippery film for drying. Instant gratification isn't always fun. It encourages sloppy burst mode photography.
3. Results. Actually, to me, this is the most important factor. I like the results of home developed b/w hybrid film photography. I like the imperfections, the grain, and even sometimes the dust. I like that the exposure isn't robot perfect. I like the way that film handles skies. I like how it handles over exposure. I'm not a die hard film photographer, I do like some digital b/w photography very much, but ... film has that romance ingrained into it. You can't replicate that with software filters. So much boring shiny sharp, magazine directed, gutless photography is created by DSLR. Okay, some pretty incredible DSLR photography around, but so much of it is unimaginative and the same. B/W film/hybrid offers something different. A different approach. It is becoming the punk of photography.