When you look at the DM's guide guide to the game, one of the very first rules it teaches is that the fun trumps being a stickler for the rules and the DM is free to bend and break rules for a better plot, and even encouraged to do so.

D&D has a strong narrative aspect when you look at the published adventure modules. There are usually plenty of characters to interact with in some way or another and some quests can be solved entirely by following the breadcrumbs offered up through them. But the DM needs to role-play all of these characters and do a lot of improv to make this work. This isn't so easy.

Also, combat in D&D is a slog. Whereas turn taking outside combat is rather fast and loose, the game turns into this enormous ceremony once the words "roll initiative" are spoken. The effect is that combat can take up a lot of playtime relative to the non-combat role playing, while often also leading to less overall quest progress per time.