Momentum.
Look at every OS that's on the market right now. It solves a problem. Right now there are very few people who can't pick an OS off the shelf that will solve their particular problem. Be it an RTOS or a fancy gui that your grandma is comfortable with, the software already exists.
I guess what I'm really trying to say is that unless you can upend the current OS landscape while maintaining compatibilty, then a new OS for the masses doesn't solve a problem. It creates one.