Probably because there isn't much of a reason to. For tourism, you could never leave your hotel, unless it also included an artificial island, in which case why not just build most of the hotel there and leave the underwater parts for observation? Cheaper and less of an engineering problem. In fact, even considering deep sea mining, it's questionable why you wouldn't just have the settlement be on the surface and use cables and tall pylons as conduits for workers and equipment.
The one advantage might be that on the sea, you can build down as much as you can build up. So a tourism based seastead or more likely, a coastal city which wants more land but reclaiming it isn't an option, might be building down on some buildings instead of choosing to build out (perhaps not wanting to mess with ship travel) or up (because they already have and don't want to build higher). But again, underwater engineering is expensive, so this would be a last resort. Perhaps bad storms would be an incentive for this.
A potential might be with Cuba as a state, where the US goes for the submerged floating tunnel approach for a combined road/rail tunnel. It's a long distance, so eventually seasteads with some sort of "exit ramp" from the tunnel pop up as rest stops. With the advances gained in underwater engineering TTL, and not wanting to put too much above water due to hurricanes and sea traffic, a lot of these structures would have undersea components which might up the majority of the buildings. Perhaps in TTL's further future, like the 22nd century, a mostly underwater city suspended between various artificial islands runs from Florida to Cuba with the original tunnel little different conceptually than a normal interstate (perhaps its designated I-95 as well), and like any interstate, has been expanded numerous times over the years. The original rest stops are now urban areas with their skyscrapers inverted, stretching down to the seafloor in some cases, while "suburban" buildings are all around them. It gets admitted as a state in this time since the million people who live there are sick of Florida and Cuba not paying enough attention to them. Such statehood gives ideas to people living around other Caribbean transport tunnels built later.
Inverted floating cities seem like the best option for underwater cities.