It probably depends to an extent on whether you are a system diehard. If you like Alternity, it hardly matters that it's as dead as dirt. On the other hand, if you play multiple systems, you have your choice of many systems, all of which are awesome, but some of which are still coming out with new awesomeness to read and use.
Mine wasn't exactly mentioned, so I will list them here.
Players: I have had no problems getting my players, that I started with DND, to play other systems and to enjoy their time playing other systems. But, yes, it took a known system (oop or not) to excite them about playing.
Support: I referenced the above quote because Alternity is my favorite RPG but lack of support is what hurt my playing it. I'm not a designer. I did use various rules and write up my own, with varying levels of success, but found that to be difficult and not what I enjoyed about the experience. Further, NONE of my players were even remotely interested in writing game rules, so it was all up to me.
This was important because, since Alternity got so little support before it was canceled, it only takes about two campaigns before you have seen 80%+ of the options in the Alternity system. I did get a little extra life out of it by switching settings (Dark*Matter and Star*Drive and homebrewed/online fantasy rules) but even then it only gave me one extra campaign. And even then, by the end of it, the players felt bored because their characters felt too similar. Even playing fantasy the same thing happened.
So, support is very important to me. Having said that, I will also agree that if I went back to play 3.X, I would have many years of support with options that I never got to try the first time around. However, there are many other issues with that. I won't mention them here as I don't want to start an edition war or derail this thread. Suffice to say that support is important to me.
Books: I think it's important for each player to have the PH or core book. To that end, I had an Alternity PH for every player. It was a bit expensive for me, although they were mostly cheap but I needed six, but I figured that was the "cost" for me of them agreeing to play the game. I was fine with that but it's a reason that I wouldn't do an OOP going forward.
(Cost side note: In fact, I just started a SW Saga game and am wanting one extra core book. They are already going for $65+ via amazon and ebay! But, regardless, I have many years worth of material to pull from before I will find that I am "done" with Star Wars Saga edition.)
So, my reasons have been stated but a) I liked the Alternity example! and b) I wanted to give my own specific reasons for a game. Being able to find support for the game, oop or not, is the most important thing to me. I found out for myself that the reason I was getting burned out on 3.x DND or earlier was because I felt I had done it all and the system had nothing else to offer me. I still had stories to tell! But no system that I trusted to tell them.