These things are great for academics or professionals in certain fields. They need ways to organize, reference, and write (or otherwise create) from a large corpus of knowledge and experience notes. They are motivated to learn to use these tools because that's they only way to manage their work.
The average person just doesn't deal with enough stuff to make it worth their while, even if they can figure it out. They may be perfectly capable of using it, but find it's too much for their needs. A person writing letters and short stories wouldn't use a desktop publishing package, it just doesn't fit their needs. Most people are content with editing their photos with simple tools rather than working with Photoshop.
The average person just doesn't deal with enough stuff to make it worth their while, even if they can figure it out. They may be perfectly capable of using it, but find it's too much for their needs. A person writing letters and short stories wouldn't use a desktop publishing package, it just doesn't fit their needs. Most people are content with editing their photos with simple tools rather than working with Photoshop.