Allowing multiple inheritence makes the rules about function overloads and virtual dispatch decidedly more tricky, as well as the language implementation around object layouts. These impact language designers/implementors quite a bit, and raise the already high bar to get a language done, stable and adopted.
It is simple to think this way, if class A inherits from multiple classes, then the class A will have the same grandparent class multiple times, this means the code will be complicated and a series of bugs will go unacknowledged. Personally, I think multiple [inheritance](http://net-informations.com/faq/oops/inheritancetype.htm) has a bad rap, and that a well done system of trait style composition would be really powerful/useful... but there are a lot of ways that it can be implemented badly, and a lot of reasons it's not a good idea in a language like C++.
