Because the language designers decided not to allow it.
There is a deeper reason for this in that it adds a lot of complexity with not a lot of "real" benefit, and the reason for that is called the Diamond Problem:
Multiple inheritance - Wikipedia[
^] covers it well.
While there are ways round this - C++ allows it - they complicate things a lot and Interfaces provide most of the required functionality, especially now they can contain default methods from C#8.0