Why is it not more popular?
Fear of unknown primarily I'm guessing. Second reason is that it still needs SOMEONE to set it up so that all familiar windows fonts, adobe flash etc. are installed ready. It's a quick 10 minute process but requires a bit of knowledge. After that it's just point and click basically.
My parents were already heavily using cloud based e-mail etc. so I didn't even need to set up an e-mail client - all I had to do was to add quick links to Firefox basically. Skype had to be installed but that was a couple of clicks away (through built-in software installation utility that Ubuntu has, you select the software, choose 'install', give administrative password and then choose if you want a quick link to your toolbar or not).
Edit: One reason it's not spreading like wildfire is obviously lack of support for windows games. That's a big no-go for families with children but also a non-issue for most grown ups.