Because software can be copied infinitely using the very computers it runs on? It's not a physical product! Yeah, I know copyright law treats software as a physical product. But the thing is, it's hard as hell to enforce. What makes Microsoft so successful in this, is they need to enforce copyright on OEMs and big businesses mostly. It's harder if you are selling to the general public.
The other major thing is, software is utilitarian. If you are some huge software company you can corner the market simply by being in a market you need billions of dollars to compete in. If you aren't, someone is going to compete with you. And one or more of them are going to competing with you using a price tag of $0. Software is not like music where we can have a collection of music of the same genre. That's totally normal with movies, or music. Maybe some people "collect" text editors, but that's not normal.