• the language is easy, very attractive for non-CS people
• the language is consistent (everything is an object, or a pointer to an object rather, you can only pass by pointer, scope and namespace that make sense all the time, etc...)
• you can learn it gradually, you can start using it even if you know only 10% of the language
• Amazing documentation. The official tutorial is easy to read, and by the time you are through with it you are fairly proficient with python
• error message that tells you exactly what the error is and where it is happening. A lot of other languages do that now, but that wasn't the case 25 years ago.
• inline help / magical docstring (`a=4 ; help(a)`)
• The REPL, especially being able to get into the REPL with your apps (`python -i`)
• batteries, again 25 years ago, no language came with such a huge standard library
• PEP 8, the grandfather to gofmt and rustfmt. Makes a big difference when working on a team
• the Zen (`import this`)
And of course as the language grew in popularity:
• the ecosystem (pypi.org)
• the amount of resources, from books to website, the answer to every questions you can think of in SO, very active and helpful mailing list/usenet group, active and helpful /r/Python