You have to consider that MNF is a one man show.
Killer7 does all the work alone.
By now MNF contains hours and hours of gameplay. And currently he is even working on a second game, MNM.
From all the work that needs to be accomplish the rendering takes the longest time.
Six ingame days in 4-6 weeks is a huge job. And he is doing that since over a year.
I don't know any other creator who satisfied his community more than
Killer7.
It's not only the "graphics". There are indeed other games with way better animations.
It's the story and the emotions that makes MNF out of the ordinary.