I question your "the rest of the land based ops ... do 3 or more".
Clearly you've tried all the rest of the land-based ops in the Caribbean and South Pacific for you to be able to make such a blanket assertion, but you really must have skipped the few that I've tried since two morning dives seems to be the standard just about everywhere I've been, with three dive trips being the exception and only done one or two days a week.
For instance, DiveTech in Grand Cayman: "Daily 2-tank boat trip". Likewise with Ocean Frontiers: "5x Two-Tank Scuba Dives AM or PM" (they only do a 3-tank "safari" on Wednesdays).
CocoView in Roatan does 2-tank dive trips mornings and afternoons, with the second tank being a drop-off on the way back to the resort.
Bonaire boat dives are usually done as single tank outings, with the exception of Buddy Dive's National Park trip. Likewise in the South Pacific, where many Tahiti dive ops will do a 1-tank AM trip and a 1-tank PM trip, though a few do 2-tank trips.
Two tank was the norm when I dove in Cancun and Puerto Morelos. Two tank is the norm that I experienced in Cabo, Puerto Vallarta, and Zihuatenejo. Two tank is the norm for the dive ops that I've used in Hawaii: Bubbles Below in Kauai, Jack's Diving Locker on the Big Island, and Ed Robinson's, Extended Horizons, and Maui Dive Shop on Maui (though Ed's does 3-tank "advanced" trips a couple days a week).
Some liveaboards will limit one to 3 dives a day, but 4 dives or even 5 are more of the norm for the Aggressor/Dancer Fleet and other American headquartered boats.
I know these must be the only exceptions since you have obviously tried them all. Weird coincidence that I've only experienced the abnormal two-tank variety, huh?