Mostly because it's bad business and marketing. It will require more manpower, longer hours and there will be loads of 'bad' reviews concerning ban waves. Also considering what you have to do it's not done 1, 2, 3 by a random person. When I first got my 2ds after I sold my new 3ds months ago I was busy from 3pm in the afternoon till 10pm at night to get it work and I still messed up with accidently updating sysnand. I'm a college graduate with a bachelor for Business IT and Management. If you're not home in the whole 3ds scene, there's a high chance you won't even get it to work in a few hours.
Our current society is focussed on getting things done fast without problems. If they can't, they stop trying. Other than that the current AH9L can still brick your system and so can downgrading. Nintendo most likely knows all about this and just think it's not worth the hassle to keep banning over and over. It takes a lot more effort to detect CFW then for example someone playing a game on the sky3ds+ using a public header.
Our current society is focussed on getting things done fast without problems. If they can't, they stop trying. Other than that the current AH9L can still brick your system and so can downgrading. Nintendo most likely knows all about this and just think it's not worth the hassle to keep banning over and over. It takes a lot more effort to detect CFW then for example someone playing a game on the sky3ds+ using a public header.