This idea would have more uncertainty for the investors, so they'd want a higher return than for a fixed bond. Taxpayers would be less likely to vote for the measure because it would have to raise their taxes even more than a standard bond.

(that being said, bond measures often take the form of "a 0.2% property/sales tax is to be levied for the next 20 years to pay off the bond", which is actually pretty close to what is being proposed by the GP. The difference is who is left holding the bag if tax revenues don't meet projections)