Dawn uses a heavily modified version of DS1's ion propulsion; while the balloon descent system demonstrated on Pathfinder was actually abandoned by JPL until the '99 fiasco suggested that active descent systems may be a little fragile for routine use....

(So the simple answer to your question is that NASA hq has been reluctant to utilise its developmental technology, for a mixture of good - as i keep pointing out: technologies like SEP don't make easy missions easier they make difficult missions less difficult - & bad - the abandonment of bounce landing research was a pure case of JPL agendaitis getting in the way of mission needs - rerasons)

Some of this research (including the failed DS2 impactor experiments) are being picked up by other agencies in Europe & Japan; & in general, the Europeans have a SMARTer (sorry) way of running these demonstrations: it's SMART technology demonstrators are specifically to linked to mission requirements; so the Hall thrusters on SMART-1 (a lunar orbiter to be launched later this year) will be reused on the BepiColombo Mercury orbiter; & a whole raft of different technologies for LISA (& Darwin/TPF?) will be tested on SMART-2.

All Japanese planetary missions are technology demonstrators with parasitical science, incidentally.

Robert