Thankfully, Autumn 2010 is proving the exception, with Fallout: New Vegas, Two Worlds Two and Arcania: A Gothic Tale 4 all due for release soon but the fact remains that compared with other genres, FPS's in particular, open world western adult orientated RPG's are few and far between.
One possible explanation is that lengthy development time limits release frequency but this doesn't really stack up as other more numerous popular game franchises have similar dev time.
Could it be that the core fan base is significantly less than for other genres so the big profits are more difficult to generate? This doesn't hold water either as Oblivion, Fallout 3 and even the seriously flawed Two Worlds 1 sales figures broke records - so there's clearly a big market here which isn't being fully exploited.
This doesn't make any business sense in an entertainment sector where serious profits can be made. When we consider all that developers and publishers need to make a buck is to produce mediocre quick turn round offerings such as appalling movie tie-ins and mediocre shooters, it is perhaps here that lies the sad & shameful answer.
Your comments & theories are welcome...