There's lots of reasons other than the H1B thing, all of them stupid.
* Tech d'jour. A lot of startups are actually under pressure to list all the latest greatest often fad technologies just to impress investors. This has been my biggest problem lately. I've got years of dedicated study to a core language that I can just stomp the competition with most of the time but oops... I don't have 3 years experience in that framework that came out 2 years ago that takes a few days to pick up.
* Lousy hiring strategy - It's kind of like what I used to observe on dating sites where people would list these absurd shopping lists of must-have requirements. The limits they put on eligible partners, or in the case of hiring, qualified employee hiring pools is absurd. Sure you might find the guy that has all of the things on your list but how freaking long does it take to pick up a newer or older version of something you already know. It's a misplaced focus on overly precise requirements over general quality of the candidate. Some people would rather have the mediocre guy with their exact bullet list than somebody smart enough to pick up near-anything you want quickly. Sadly, I met my wife by getting into a fight with her on Craigslist but this hasn't helped me out with any jobs yet.
* Incompetent leads running the show. We've all met them. They don't want to learn anything after college that they don't have to. Why they stayed in tech, I have no idea but they're out there and sometimes it's just easier to scoot them up to management where they still feel like they have to have their own tech limitations lowering the bar so they can understand what you're doing. Even if it's not really their job to be nosing around in the specifics.
* Lazy HR. Some people will always see it as a resume pile culling process, even when the skills they're looking for are rare or hard to come by in certain combinations. The more reasons they can get to toss somebody in the 'nope' pile, the fewer people they ultimately have to talk to/deal with.
* Lazy teams not policing the HR ad copy. The HR people don't know anything about versions. If it's a clueless hiring manager that's to blame the blame should also fall on the tech people who didn't stop them and tell them that getting overly explicit wouldn't be necessary.