I am not sure why everyone thinks SM is so crazy or for cave only.
(All of the following is written from the tourist operator perspective, and not from the diver perspective, because my experience is completely from the tourist operator side, and not from the diver's side.)
Operators (probably) don't want the traffic at the back of the boat that sidemount boat diving (seemingly) tends to create while tanks are being handed into divers in the water, or handed up at the end of a dive.
Tech boat dives are similar in that the slung bottles are handed into divers in the water or handed up at the end. With tech diving, there is a decompression based reason to not have divers who have just finished divers working hard to get their doubles and the sling tanks back on the boat.
It is easier for the operator to accomodate general needs rather than specific needs, and it is easy enough to say sidemount is tech because sidemount is often like tech in practice, as far as the 'on and around the boat' behavior is concerned.
For most operators, using a single 80 forces divers into being conservative with their dives. The captains I know prefer to have divers limited by their air, and don't want divers carrying extra tanks because too often the largest tank diver makes everyone else wait at the end of the dive, or goes into deco.