Keep in mind that the cruise ships do not own the equipment or companies ashore that provide excursions. They contract with them. If there is no accessible bus or tour available in the whole city, area or country, how do you expect the cruise line to provide one out of thin air? Even in USA ports, where the ADA is poorly enforced (such as Hawaii), you will often not find accessible tours. In 3rd World countries without an ADA equivilant, and with disenfranchised, universally poor disability population it is not going to exist.
Yes, the cruise lines should be more pro-active in this area, but frankly I don't think they really want the disability business and most would rather we did not come on their ships at all. When we do, it is more work for their crew, etc. Too bad for them, we are a significant part of the market. The reality is that they will not change without constant pressure and until they are forced to (just like access in the rest of the world).
Given that, we don't like ship's tours anyway. Who wants to go with 80 people in a big bus to see beautiful sights and quite beaches??? The ship's tours usually range 50-100% more expensive than trips on your own. We would much rather arrange our own excursions, and find that this is usually much less expensive, meets our needs, and is more flexible. I get tired of ships tours that steer you to their preferred shops that give the ship a kickback, often have higher prices, etc. We have been all over the world, and never had any problems (with a little advance planning) arranging independent tours, and have never missed the ship.