Because agencies specialising in licensing a patent (i) are non practising entities, and many people's view a troll is pretty much defined by the concept of acquiring IP without the intention of doing anything with it other than seeking fees, and (ii) occupy their time sending legal threats to firms already using aspects of the patented invention rather than delivering sales pitches to people searching for technological breakthroughs, because "the right to develop your own technology which does X using Y" is a really hard sell.