Because there was a huge push by the Python designers towards this direction?
They got funds from the DARPA in 1999 for a proposal entitled "Computer Programming for Everybody", the first part of which was " Develop a new computing curriculum suitable for high school and college students".
The Python project did a lot of outreach and marketing specifically targeted at educators to explain why Python was a great teaching language (and did a lot of work to make it so - don't get me wrong). It worked.
Javascript is quite popular as an introductory language too -- students can open up a web browser and type stuff in the console in the middle of a lecture.
Also, Python is used as a scripting language on top of C, and C is popular.
Different domains of education had different reasons but some classes had nothing to do with Matlab.
- The professor Gerry Sussman of the famous MIT 6.001 SICP class said they switched from Scheme to Python because (paraphrasing) it's more high-level with libraries to get immediate work done. (E.g. A class project to control a robot.)
- Peter Norvig teaching Artificial Intelligence classes switched from Lisp to Python because he noticed his students kept getting stuck on Lisp syntax instead of progressing on the more important AI concepts. Switching to Python made teaching the class easier.
One can google for their interviews on why they switched to Python.