The problem at the moment is that most people are hoarding their coins, because the price has dropped significantly. Everyone does
not want to be the "Pizza guy" in a few months when the price goes to a ATH again. When merchants start to accept Bitcoin, they
want people to use it, not hoard it. The solution will be for people to have quick access to Bitcoin ATMs to enable quick conversion
from cash to BTC when they enter these merchants. {You have the cash, but you want to spend it via Bitcoin}

In connection with McDonald's and other food or other giants on the market, I have a different idea:
They are big enough but not dumb, this is why they are making such profit now. They have a staff that can tell them if it worths for them to accept bitcoin or not. If they will have the green light for bitcoin, there will be a big marketing campaign about bitcoin, so we'll see it immediately.
My idea is, that they will start to accept bitcoin if they will think (based on their own research) that the price of bitcoin will raise and for a longer period. This will let them to start a new marketing campaign with bitcoin and they are big enough to start gathering bitcoin (and not giving it back to circulation). Accepting bitcoin itself won't make them significant profit but gathering bitcoin and waiting for it's price skyrocketing will make them the expected profit. As they are big enough to finance the amount of bitcoin income somehow differently, they can make much higher profit in the end.