@mirazu
Likely due to marketability. Adult anime don't sell as well as far as I know, fewer air slots, etc. When a producer is looking to pick up a series they're usually gonna be aiming for manga marketed towards the 10-18 crowd.
As you said, there are exceptions but there's just fewer slots to fill so competition is gonna be more intense. This series has me hooked, but even I will say it's (so far) not one of the more memorable series in it's genre. There would be better series to adapt into anime.