The danger with multiple inheritance of concrete classes is that there is storage and virtual method lookup that must be reconciled between the two or more parents of a given class. Especially tricky is when there are shared ancestors. But interfaces only define what a class should look like, not how it needs to be implemented and it's much easier to make a class look like a lot of different things than it is to make it be a lot of different things. Two interfaces can require a method int Foo() and an implementing class can safely use both interfaces and implement Foo() without causing headaches for which base Foo() to override, etc.
Another reason is that constructor chaining is difficult to manage with multiple inheritance. But interfaces don't specify constructors, so that problem is entirely sidestepped.
There are certainly many other reasons why multiple inheritance is bad.
