Because most of the time, dev teams and/or publishers are so busy with what they're currently doing that they forget all about their past games. Some of them do remember once in a while of course such as Raven Software who somewhat recently released the Jedi Knight: Jedi Academy source.
Let's not forget that opening the source code or/and making Unreal/UT free has some implications relative other parties, such as Steam for example, who still sells the game (along with some others).
So in order to do that they would have probably to allocate at least 3 people: a lawyer, an executive and a developer to handle that (I think, don't quote me on that).
Furthermore, given their excuse of not opening the engine until a few years ago, that excuse is still quite valid in what concerns at least UE3.
But, the fact that Epic made this move with UE4, certainly makes the opening of UT/Unreal more plausible than before, and yeah, they wouldn't loose anything at all with it, as no one in their right mind would try to bring UE1 to the level of UE3 or even UE4 instead of using UE3 or UE4 directly, the amount of time and effort simply wouldn't pay off, and that's probably even why Epic is betting in this new model, they know what they have in hands and the fact that no one else comes even close to them in this kind of technology.
Still, many would be very happy if they made UT free and open source (one of those is me of course, I would be overjoyed).
I just don't go ahead with UE4 right now for my own stuff (yet) since from the description the whole thing is still it's early beta phase, to not mention that it needs a beast of a machine to handle it properly, at least for now, other than that, I am really looking forward to the next developments concerning UE4.
Or we could get a bunch of guys together from various forums and call it the UT4 Development Team. I would only be able to review and look at stuff though.
