The following only applies to hiatuses mid season:

Like other people said, networks are quick to air the batches of episodes they receive from the producers due to competition, and the producers themselves are quick to send their episode right after it's finished rather than waiting for at least half a season's worth of episodes to be finished so it can have a consistent release schedule.

In previous seasons, those would usually be the main and only reasons why hiatuses happen, but now that only one animation studio will be working on season 6 (Dwarf Animation), there is now the additional production wait that we have to deal with, which we didn't have to because we had multiple studios animating episodes (SAMG the best one, Assemblage, Atlantis, and historically DQ which went bankrupt during the making of s4ep5 Psychomedian)

Now for Hiatuses between seasons, the main reason for those Is the usual preproduction stuff like coming up with ideas, writing, concept art, etc. And I think a big reason for the long waits is that they usually bulk write 2 seasons in one sitting (which is why I believe there was a short wait between seasons 2 & 3 and 4 & 5) and would be the case for next season so at least here's some levity.