As I was saying, a general scripting language like C# creates more freedom for every developer, they are not limited to the possibilities of the scripting engine. Secondly, just because the fact that C# is that general, and known by a lot of programmers, means that programmers that are already capable of programming using the language, do not need to learn a new language and are able to use all the experience they got with C#. This cuts down the learning time and costs for a developer; should be an easy choice. (Or you have some programmers in-house that have experience with Unreal Script of course, but that's less likely to be the situation)