well in my opinion the main reason is not the volume you speak of. i feel like bitcoin has enough popularity its just that buying into bitcoin is somewhat complicated. theres so many verifications on exchanges that need to be done. for a anonymous currency this is quite the opposite of ideal.
It's not more difficult to buy bitcoin than it is to pay with paypal. You create an account on one of the exchanges (just like you create an internet bank account or a paypal account), you wire the money from your bank account, and you press the "buy" button. This can't be the deterrent! Millions of people are using online banking every day.
There are other real factors that make bitcoin difficult to understand for the masses, like the private and public keys and the fact that the coins are your responsibility. You can't insure your wealth or run to the bank office for help. If you lose your key it's game over.