Probably the lack of motivation, People don't do things for no reason at all, There is currently no motivation to use bitcoin generally and as soon as there is a positive stimulation "like discounts etc..." people would use it.
There is definitely some of that, it's hard for people to change their ways when there are no incentives or obvious benefits to motivate them.
But I still think the main problem right now is volatility. Lay people won't even consider it as a replacement for paper money because the price fluctuations can be extreme and are way too unpredictable. When you have to pay bills or feed a family, you can't afford to not know how much money you have in your pocket.
The time you take to enjoy a beer in a bar could be enough for the $100 worth of Bitcoin in your pocket to drop to $85 or less. Now imagine you're in a restaurant, you've enjoyed your meal and when you pay your $95 bill and tell the waiter to keep the $5 change, the guy says hmm I'm sorry sir but you've just given me $85
