There are other types of mashups as well. Books like Pride and Prejudice and Zombies and Abraham Lincoln: Vampire Hunter take existing materials (Pride and Prejudice or Abraham Lincoln's biography) and combine them with a different genre (in this case, horror). There are even movie mashups:
So why are mashups so appealing? One theory is that they do things that are unexpected - they are uncanny, to use the term Freud used to explain humor. This could be part of it, but at the same time, certain mashups that are really well done get my attention and can sustain it. That is, I can listen to a really good mashup many times without it losing its effect, while humor - on multiple viewings - loses some of its effect.
Another possibility has to do with how your brain reacts when encountering the familiar versus the unfamiliar. Last year, I blogged about travel, cognition, and the balance between feeling comfortable in the familiar and the stimulation of the unfamiliar. Mashups, especially when you are familiar with the original source material, represent a middle option between familiar and unfamiliar. The sections containing chunks of a single song feel familiar, but as you settle into cognitive autopilot, the song shifts, activating your attention.
There are many great mashups out there on YouTube to check out. I've even been trying my hand at a few recently (and I may even post some here when I finally finish them).