I think the main reason why a shop doesnt want to accept bitcoin is its violatility... a currency that can go 20% up or down a day is not a good payment method for them, but on the other hand the more people accept bitcoin the less volatile it will become as the market will groß and the supply and demand wont be seasonal... but this is only theory. On the other hand most small shop users dont know how to do handle bitcoins so there should be a payment gateway for them to easily accept bitcoins which are converted in the actually market price to whatever currency and that goes to his bank account... an easy solution is always a way to go for mass adoption.