Because frankly, expert programmers don't want to waste time tending bonsai when they're experimenting. That means they want a language that helps them get prototype-quality results fast. That's why languages like perl and lisp are so damn powerful: Their design makes it easy to get results fast.
When Python makes you spend so much time on some code aesthetic that you're not experimenting, you necessarily end up with a second-rate design. Oh sure, it looks pretty, but it took you three times longer, and it's slower than what could've been accomplished in better languages.