Most proprietary software strongly encourage you to stick
with them, and make switching difficult. The best examples
I have in mind are operating systems and office suites.
Most people aren't as cautious.
The company I work in currently have to buy and use MS Word 2003 because the government agencies it deals with want .doc, period. The sysadmins are forced to use windows because that's what most people use, and they just won't switch. (And we're a tech company, so imagine the others.)
Inertia is enormous. People tend to stick with whatever they started with (Blender vs 3DsMax is a good example). Proprietary formats and non standard APIs make it worse.