Even though other languages handle multiple inheritance without any
problems they each handle it in different ways which can lead to subtle
bugs when moving from one language to another. Some languages have even
changed how they handle multiple inheritance between versions (Python
springs to mind here, multiple inheritance got 'clarified' at one point -
or so my memory tells me). There is only one way to do single inheritance
and this is a good thing.

Coming from languages that allow for multiple inheritance I must say I have
yet to encounter a situation where I have said "this would be so much
easier with multiple inheritance"