Jeff,
I can tell you why, from my perspective, which I think is the same as many other divers'.
Your dive shops don't advertise the availability of tanks w/air/Nitrox and weights. They don't have a "package price" for a week's worth of diving. In fact, my research earlier this year yielded the fact that none of the dive shops I found on Google would even let the tanks off their property (i.e., their house beach/reef) without a guide - either shore or boat.
If I found a shop that would, like Bonaire, let me take 2-3 tanks per person, and go to these shore dive sites, and then return them for full tanks, at a reasonable price (I don't want to buy the tanks, only use them for a week), I think that many of us would be all over the deal. Dive Bus was the only place I found advertising shore diving, outside of the local house reef. Most advertised that you were not allowed to take tanks off the Island, due to thief problems. While I understand their concern, especially since it's a larger island and possible residents/long term visitors might be tempted to "keep" the tanks, those of us who are there for only a week or two, aren't going to try and take them off island.
Maybe you could ask some of the shops to let us know if they have rental tanks available to vacationing divers to take on shore dives, without a sponsor keeping track of them?
Thanks for the information in any case...it does motivate me to "look deeper" into this for a future trip.