Why was UNIX rewritten? What computer language was used?

The earlier versions of UNIX were written in Assembly Language. That was a great accomplishment for that period of technological history but Assembly was tied to the instruction set of the microprocessor of the machine it would run on. You can imagine the headaches that caused. As hardware innovations increased at a rapid rate, that wasn't feasible so later versions were written in C++. In short, technology wasn't just developed, it was an evolution and UNIX played a big part in it.