it used to be the price, primarily, but I think there's best of the old school burger places (e.g., not In 'n' Out)... When I want a borger I go to a local joint though. I think they're even cheaper than McDonald's now.
I can go to a breakfast place down the road and get the most delectable hand made chicken sandwich with cheese, lettuce, and tomato for exactly $5. Only downside is they close at like 3pm
Look sometimes you just want to order a 40 piece nuggets and be in and out in 5 minutes. It also helps that I work 3rd shift, and it's like the quickest place (and one of the few places still open) for me to get something to eat on break if I end up sleeping all day before my shift.