Axas and dotnet are right, just imagine the mess it would be with multiple inheritance.
in my opinion, it just makes no sense. During all the time ived been developing in c++, never have i used multiple inheritance. why? Because the maintenance of the code is a lot more difficult, understanding the code is harder, and whole lot of programming difficulties it implies, and the most important reason, i just never encountered any real need to use it, besides if you ever search up multiple inheritance in c++, theres is no real example that proves it useful.
Then i started developing java apps and found that interfaces where alot more practical and useful, because you can multiply inherit the interface, but not the imlementation, wich was up to the client class. wich simply does make sense,
code looks clean and understandable.
So as a conclusion. I think creators of c#, really made a noticable upgrade to c++, added some stuff that came along with java that simply seemed useful(and other new stuff that .net implies being .net language) and voila, resulted in c#