Because multiple inheritance is complicated and not needed. One problem that multiple inheritance introduces into a language is the
diamond problem. In C++ this was solved by giving you both options: the default is to have separate instances of the contained superclass, and with virtual inheritance you can have a shared instance of the contained superclass.
In practice, multiple inheritance is never really necessary - I haven't felt the need to use it in the 8 years that I'm developing software in Java.
So the Java language designers chose to not make Java unnecessarily complex and left it out.