C.S. Have you been to Xcalak by boat?
A few years back about a mile north of the Mayan cut that seperates Ambergris Caye from the rest of the Yucatan Peninsula the Mexican Government dynamited and dredged a hudge channel from the reef side through to the Bay of Chetumal.
Before the channel was made a bridge even smaller than the one that is being built at the cut now is all that would have been needed to access Mexico. Now with the channel in place two bridges would required and the second bridge would be a major undertaking.
I doubt that Mexico would have much desire to put this bridge in. If they did it would certainly make life easier on the north end of the Caye.