Even if installing Ubuntu is easy, Linux is still confusing for the average Windows user who has spent years trying to understand Windows and Windows-based software and now must re-learn where and how to install, configure, and use software to be able to be productive with Linux. All the same software is not available on Windows and Linux so the user probably will not even know the name of the software that he needs to use to accomplish things that he already knows how to do on Windows.
Re-learning takes time so a user is going to need a very compelling reason to do it. Every time the user has to go to a search engine to figure out the name of some software he needs to use that he already knew on Windows he is wasting time that he could have spent doing something productive if he did not change. What is the reward of all this time spent re-learning Linux instead of Windows?
The things that Linux does better than Windows are all related to either open-source software-development, security, or server functionality. None of these are important to the average desktop user, so the desktop user has no compelling reason to use it except maybe cost, but even if you have to pay $300 for Windows it will ultimately be cheaper and less hassle to use for experienced Windows users because time is valuable and it will take a lot of time to re-learn everything.
The desktop user has a lot of compelling reasons not to use Linux: he already knows how to use and install Windows and doesn't know how to use Linux, Linux is incompatible with important pieces of software that he wants to use like Microsoft Office or the latest PC game, most of his friends use Windows so if he has trouble he has people he can ask for help or advice, but he has no such support if he uses Linux, and all of his hardware came with drivers for Windows but he may not know how to find or install a driver for the same hardware for Linux.