The mental burden of having to learn something entirely new (without much guidance) can be frustrating - especially if you're forced to make the switch. Think about the last time you learnt a new programming language that was very different from what you're used to. E.g. going from a C style language to a LISP, or to something like Scala from C with no background on the JVM.
For most people, the thought of installing Ubuntu or whatnot would not cross their mind if their PC/laptop came with Windows. Nevermind that the average user just knows how to turn a PC on/off and use a few programs/websites. One reason why I think the LiMux revolution didn't sit too well is that the precedent for user experience nowadays are set on mobile platforms where everything is extremely convenient and require little or no configuration - it just 'works'.
Linux systems on the other hand tend to require a lot of intervention/hand-holding (e.g. chmod) and rarely come with wizards to simplify processes (e.g. try installing an unsupported USB WIFI adapter). The world would be a different place if kids grew up using Linux instead of Windows :). For that reason I vowed to teach my son some Linux basics and a few programming languages when he becomes older.