Learning new applications is hard. They know the word processing keystrokes, spreadsheet menus, and graphics commands like the back of their hand. Switching to another office suite, like Sun StarOffice, is very difficult for most people. All of their existing documents, representing years of work, may import poorly to the new software. Key features that they depend on may not be present in the new application.
People also are busy at their jobs.
They don't have a spare 10 hours to learn the basics of some new software, or another 50 hours to get really good at it. For Linux experts, learning a new application is relatively simple. For most computer users, it is not. Users are incorrectly resistant to change, the argument goes, when they should be accepting something that is new and better. This is backward thinking.