Operator overloading + great interop with C - the ultimate cross platform (and most popular) language

Great standard library (Javascript still doesn't have one). Even better than Java's

Cross-platform

Interpreted language with minimal environment needs where just having python.exe is enough (compare build systems C or Java, or environment setup needed for PHP/perl)

Great DX given how simple core syntax and standard libraries are (again, compare with perl magic, or PHP madness)

Early numpy/scipy packages found amazing uptake in science community which acted as evangelists. This happened to great DX (scientists don't like coding C) and great C support (they still need C speed)