Because programming in Python is great for productivity, has lots of useful libraries, is rather bearable/enjoyable depending on your view point and for 90% of people/companies you don't need the speed of anything more than PyPy or Cython because, believe me, you're not at Facebook or Google scale, and even those companies often use Python (Google most famously).

Btw, for those of you who dislike the dynamic typing (e.g. myself) of Python, check out Cython, it's really heaven.