The OSR movement is growing for a few key reasons. One is sentimentality towards the roots of the hobby. The other is, I think, a natural backlash towards "heavy" systems. There's also a final point I'll try to make but I'll let others debate it.
Most of the people who were in their teens during the really big growth of Dungeons & Dragons are all now at a point in their lives where nostalgia has firmly taken hold. (The 30 year mark is typical.) If you look at the market there were, originally, very few products in the OSR mold a few years ago. Nature abhors a vacuum and *poof* that vacuum is now being filled with great products aimed clearly at the demographic looking for something similar to what they grew up with.
When you look at the largest dog in the RPG kennel right now, Pathfinder, you find a great deal of what the OSR movement espouses to not be. There are easily a dozen hardcover books that one could own to cover almost every possible scenario, class, race, etc. While none of them are necessary and any GM could simply nix their use they exist and if this hobby is one thing it is rife with completists. I don't think Paizo has set out to crush bookshelves or curve spines by publishing vast tomes but scope creep occurs and both they and other publishers gradually lose sight of their audiences as the forest gets thicker.
OSR is, at heart, a chance to go back to where things occur and instead of consulting Volume XXII of "UBERGAME!" you simply roll a die and move on. It's about the story first and foremost and not about who has the disposable income that allows them to be the one to have the most rulebooks.
All role playing games should emphasize storytelling and adventure. Unfortunately some of them lose sight along the way in forgetting that a game should never be just about the rules but more about the players participating in the adventure.
The last point I would make, which will no doubt spark rage, controversy, disagreement and castigation, is that OSR is, for the most part, fairly agnostic on a number of social issues and "adult" themes that have crept into role playing games over the last decade or two. OSR seems, at my glance, to throw much of this aside and gets back to the core of our industry roots which is, in the simplest of measures, killing monsters and getting treasure. There isn't an in depth examination of the motivation of the Kobolds, their mating habits, their bonding due to environmental and external influences, their societal roles, their relationship to the greater dungeon ecosphere, whether their desire to wear no armor or full plate is a manifestation of their repressed desires to emulate bugbears or any of the other things that other systems supply in mindboggling detail. They're simply a roadblock to, as the kids say, more phat loot.
We are all older and wiser than when were kids. (It happens.) That said as we grow older, have families and take our parents and grandparents places in life we look back on what we loved and cherished and move to have that in our lives again. I watch it with my friends and I watch it with my own son. I think it's natural.
We've no shortage of forkwits in this world who will want to tell you how to enjoy your games. They delight in telling you that you aren't doing it the right way or that how you feel is incorrect or that what you love isn't nearly as sophisticated as what they are doing.
Bluntly, screw them.
Go roll dice, laugh at your failures, mourn your characters wounds and wind up with stories as good as the ones you told your buddy when you were walking his paper route 30 years ago.
