Because for developing software, there are two performance criteria, one is the performance of the code, one is the performance of the people developing the code.
Yes, python may be much slower than languages like C/C++ etc, but it has a lot of libraries and its simplicity gives the developers a more productive way to deliver software in a much much shorter time. And for most of the tasks like website etc., network latency and IO cost much much more than the evaluation of the code.
That's why a lot of websites nowadays using languages like Python, Ruby on Rails, etc. because they give the developer better performance at developing the product, write shorter, more maintainable code.