The vaguely come up with a plan for all the expansion's patches sometime after the .3 patch of the previous expansion, and they simply did not plan on another deep dungeon because they thought people didn't really care about them that much. Most likely they noticed a massive drop in activity in Heaven-on-High compared to Palace of the Dead, or something like that, because of how more people have access to POTD than HoH. Remember that back then, trying to get the Necromancer title in POTD was not as big of a thing, and you only started really seeing people showing off the title after streamers started doing it and created interest in it some time before Endwalker.
Deep dungeons were part of the "try new ideas to keep things fresh" part of the content cycle, so once people seem bored of it they try something new and so instead we got the firmament events, ocean fishing and bozja CEs, two bozja CE raids, an extra extreme and bigger raids such as DR/DRS.
