High level D&D is a different animal than low level D&D. It is, frankly, a lot more work for the DM (and for the players, for that matter) than low level D&D is. It doesn't much resemble traditional fantasy adventures but is closer to magical superheroes. There's a lot less to do at high levels, especially in fifth edition, which doesn't do much to fold in new challenges like domain management (which plenty of people, including me, found boring in previous editions), meaning that, eventually, you end up just a little weaker than the average demigod and given the choice of fighting the same sorts of challenges your last high level game included (different gods, different fiendish leadership, etc., but similar kinds of problems), probably bouncing around the planes, etc.
In contrast, there's a much larger variety of games you can run at low and mid-levels.
If you wanted to run a campaign resembling Game of Thrones, high-level player characters are going to fast-forward right through it (and more than King's Landing will be going up in flames, so I hope you don't want your setting to remain intact for future use). Likewise, Lord of the Rings will be a handful of short adventures at most. Even the War of the Lance will be relatively brief, although it'd last longer than the other two examples.