i'm just not interested in modern bikes. too much plastic, electronics, computer controlled stuff, often too specialised... so that's just personal preference for something that, if push comes to shove, can be fixed on the roadside in the rain with a hammer and does everything reasonably well and reliably.
that's why i've ended up with bikes from the 70s or early 80s. never seriously considered anything more up-to-date and sophisticated. actually, i never even had 4 cylinders before and looked long enough at the schematics of these bikes before concluding that i could manage that, and went for it.
after i moved to amsterdam i didn't have a bike for a while. then i got and restored a honda ss50 moped for commuting - couldn't see myself riding one of those modern scooter plastic buckets. soon i became itchy for something bigger, though, and stayed with the honda theme - that's where the 550 came in which is basically a toy for me, not a necessity like the moped.
i have this vivid childhood memory, probably romantically tainted and distorted at this point, of sitting next to my father in the car, standing at a traffic light. in front of us was a bike with a distinct chrome fender and tail light. the light changed to green, the bike took off and was gone but never left my imagination. i was too young to ride or drive anything at the time but now i recognise that rear as belonging to an SOHC4 honda which somehow since then has become iconic to me. nothing rational, just a dream but hey, i have now the time and money to live it so why not?
there's also the social aspects. wherever you stop with these bikes there's people coming up to you telling you stories they had back in the day with their bikes or asking questions about it, getting into a friendly conversation... try to have that kind of interaction riding a crotch rocket.
why the hipster crowd is getting interested? i don't know and i don't care. i suppose once they realise they need to search for parts and can't just pop down to any dealer, and have to get their hands dirty and can't just take it to any shop, they'll drop them soon enough.
why the prices increase? well as time goes by these bikes will get more rare, especially in stock configuration. that naturally makes them more precious.