Well, having made a computer with an AMOLED screen (Atomic Cobalt1) I can say we would love to be still using them, but manufacturers stopped making them in the size we wanted- about 2.4" diagonal. Well, one manufacturer still made them, but they would not sell to you unless you were committing to ± 200,000 annually. Watch size screens are now another story, as they are becoming more available due to smartwatch demand.
AMOLED displays, given good interface design, actually use less power, as the pixels that are not lit (black) use none at all. That's more of an issue the bigger the screen gets. We lost battery life when we had to switch to a TFT display. The AMOLED displays were considerably (several times) more expensive, but had great benefits.
The diving market is incredibly tiny. It's in a completely different economic world than other consumer electronics. Parts and assembly cost more due to low volume, and design effort is amortized over a small number of sales. Molding tools & other tooling also need to be amortized over a small number of annual sales. For most computers, I suspect assembly and labor are the costly parts. I know if we could have sold the Cobalt for less we would have, but it's just costly to manufacture small volume products.
Coding a decompression algorithm is not difficult, but designing a dive computer system that accounts for an effective user interface and anticipates all of the unexpected things a diver might do, and does so reliably in real time, is not trivial.
Ron