A massive body of existing code that you can leverage.
I keep putting off picking up PHP, but when you look at the number of off-the-shelf easily-extensible solutions for CMSs, CRMs, and e-commerce platforms, there's no contest. PHP wins.
If anyone can point me at a Python e-commerce solution that will let you deliver an MVP in an afternoon, and your first original feature within a week, that would make my month.