Hi, Marty. I'm no expert but I think cost is a bigger factor than fuel. Most boilers can be changed over from gas to oil or vice versa just by changing the burner. With boilers though, you treat the water heater as a separate temperature zone, with all it's attendant piping and controls. With a gas water heater, you just have to run the gas line and there are so many gas water heaters made that they are relatively cheap.
If you really wanted a gas fired boiler supplying your hot water, I'm sure that any competant HVAC contractor could find one, but keep in mind that the boiler would have to be larger than without supplying the water heater. Don't forget, too, that you can shut down the boiler during the summer if you don't have AC. Running the boiler just to supply hot water gets expensive. Really, it makes more sense with a gas system to have a separate water heater.
Hope this helps, or at least gives you something to think about. Henry in MI