There are other, ostensibly more legitimate reasons. Most DMs know that they want to engage their players. Some DMs think the easiest way to accomplish this is through the setting. A dark, gritty world comes off as "edgy" and more engaging than another high-fantasy generica. In departing from stereotype, its easier to make an impression on the players. Under this logic, high fantasy is arguably the harder of the two modes to run.
I would argue that high fantasy is most definitely harder to run for a DM for many, many reasons. First off, high fantasy means world threatening plot lines (this being the primary distinction between high fantasy like Tolkien and Sword and Sorcery fantasy like Conan).
This in turn means you have to create plot lines where you have a world (or at least really, really big) threatening threat, someone behind that threat, all the way down to the local threats the low level party can deal with initially.
That right there is a bucket load of work for the DM.
High Fantasy also does not lend itself to episodic adventuring, nor does it lend itself to sandboxing. In episodic, you don't have a large overarching plot, thus, no world shattering threat, and in sandboxing, you again don't have overarching plotlines.
Let's also take the point that in high fantasy, most people equate that with high magic as well. Magic is powerful and fairly ubiquitous. All sorts of scenarios get nerfed when the wizard can just wave his hand (or the cleric for that matter) and solve the problem. Starving in the desert after your caravan has been attacked by raiders? Poof, create food and water. Poof, overland flight. Poof Teleport. End of grim and gritty problem.
Need to search the jungle infested by poisonous snakes, diseased creatures and whatnot? Poof, Find the Path and problem solved.
To me, it's totally not a shock that DM's would prefer a grim and gritty setting. Using Doug M's definition (which I feel is pretty common), it's much, much easier to DM that sort of setting.