It might have been that initially. These days it’s because a whole ecosystem was created around it, and yes, most of it it’s kind of free.

Think about it: if you wanted to work on something but you had to pay either for the software or documentation you need to base your work on, it’d be a blocker of sorts for a lot of people, specially if you’re just a student wanting to do a “simple” thing or wanting to test something. That’s just an example.

Having a 30-day trial of something helps, but it’ll probably not give you all the answers you need.

There are many more operating systems that are free (both free as in freedom and as in free of charge). None of them became as popular as Linux.