Because of the D&D Next playtesting.
In a nutshell people are finding that a D&D Next playtest campaign works best when you manage it in the same manner as a classic D&D campaign. Starting with last year's D&D Experience, classic style gaming has been reintroduced into a far larger audience than myself or any of the OSR publishers could have done.
Already my long tail sales of Majestic Wilderlands and Blackmarsh have shown an uptick over the past six months. Before I was getting maybe 4 or so, now I am getting 10 or more sales a month.
The AD&D reprints are just the icing on the cake. Not only the gamers will get their hand on the original books but also there are dozens of products readily available for those books.
Now I am realistic about this. Victory doesn't mean that OSRIC, Labyrinth Lord, or Swords & Wizardry, or any other retro-clone is going to compete head to head with Pathfinder or D&D Next. The impact of this is that the classic editions of D&D will have enough gamers behind it to keep it a living game with a vibrant community supporting it for a long time. Which to me is the point of all this to begin with.
And if it turns out that D&D Next continues to be compatible with classic edition D&D, well that just makes it easier. And to keep D&D Next compatible is the main reason I am involved. Not just because I like older editions, because that means all the stuff I wrote will be usable as is for D&D Next gamers. And for you OSR gamers reading this this means your writings too.
The point of the original retro-clones was to get people publishing and playing classic editions of D&D. To preserve them for the next generation of gamers. That goal has been achieved and we are in the midst of a new golden age of classic D&D.