Most of the reasons have already been touched upon by others:
1. Ease of finding players/GMs. If a game is currently in print, then it has more exposure and so people are more likely to recognize it and be interested in trying it out. Also, in print books will generally be cheaper than out of print books, although this can vary widely depending on the system.
2. Ease of acquiring the books. If a system is in print, the books will be easier to find, which makes it easier to acquire an extra copy of books to use at the table, or for players to acquire their own copies.
3. Current support, in the form of both supplements, updates/errata, and fan-produced material. A system that is in print will have more supplements and the like coming out, which can be mined for ideas even if you don't directlly use the supplement. Also, being in print means more exposure, which means more interest in fan based modifications, ideas, and scenarios for the game. Note that particularly popular out of print systems can have more supplements and fan-based support than many in print systems will ever reach.
4. In print systems are more current and therefore benefit from experience over that time period.
Personally, unless the books are particularly difficult to acquire and/or I do not have enough of them, I consider "in print" to be only a minor bonus. Several times, I have purchased an entire game line after it has gone out of print (Deadlands Classic, Fading Suns, and Unknown Armies are a couple of my favorites that I did this with).