If I can find players for an out of print book, then no problems, but usually it is. Most put of print rulesets are pretty obscure, and get more obscure, tough D&D is an exception to this. Everyone knows D&D.
Trying to find old rules is just difficult.
Player pool is the big one for me. "Well, go look on eBay for a few weeks and keep an eye on Noble Knight" does not a healthy pool of players make.Available player pool, current support from the company, additional support by way of articles or regular discussion from third parties, etc., IMO.
Oh! And if it's in print, you don't loose it when your hard drive crashes...

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.