Why does there seem to be a dearth of interest in solar sail technology for interplanetary travel?

Solar sail is fairly useless inside Mars Orbit - the distance isn't great enough for it to have time to be effective. Where it will be useful is setting up long orbits that look at outer planets, comets and the Oort Belt. Then the very weak pressure of the sun can be applied for a long time and build up velocity - along with sling shot boosts from the planets - that can set up interesting orbits. Of course, once the vehicle has been propelled in this kind of operation, the sails have to be folded or abandoned so it can return back to us.