Perhaps partly a lack of repaint opportunities (the Battlechargers spring to mind, but they're hardly prominent) and a lack of memorable characters. Aside from Hot Rod/Rodimus, Magnus, Grimlock, Galvatron & Cyclonus who really got much attention in the media? A lack of a Galvatron is surprising, but I'm not surprised we've yet to see a MP Blurr. The 1986 Decepticons, BTW, were actually very sparse once you take out the combiner teams.
Personally, I'd LOVE Masterpiece Stunticons & Aerialbots (also appeared in season 2 of the cartoon, so from a character POV I lump them into 1985) and would bite your hand off for a Masterpiece Pipes... but Pipes was hardly prominent and I know he'd sell poorly. Even if I'd probably buy two
