Last few days at work i was thinking about this question.
I'm watching for the storyline of those games nearly since beginning (as i started playing Unreal in early 2000).
Through the years, there was created uncountable amount of content for both of those games. Maps, mods, gametypes, mutators, total conversions, some of them silly, some of them - extremely popular etc..
In the middle age of this story - Epic's gived a chance to improve this game a bit, leaving it's original spirit and gameplay. With their decision Unreal 227 patch was born, which gave improved stability, network sequrity and much more possibilities for creating custom content. And i really liked it.
That was a small synopsis for the newbies which may appear.
So what is the actual problem to make one game from two games? Network compatibility? Version difference? Difference in native code? Content creation?
As for network question, i was rarely confused when i'd found an Unreal coop server somewhere at UT DM server tab and was able to connect there and play coop, actually playing Unreal through UT without installed oldskool mod or something that makes this possible to be

I just downloaded maps and some custom content and played as usual.
This topic is not about words kinda "this game is over 100500 years old", "noone will play", "UT4 is coming", "i prefer to play Battlefield\UT2004\UT3\UT4 or whatever", "this idea doomed to fail" etc..
So please..
I want to hear your thoughts fortified with facts.