I think possibly because it seems out of the people who join Atkins, only a small number of them progress to OWL, and even smaller to Maintenance.
Basically most people see Atkins'ers at Induction, which again only few people actually consume 20g carbs a day. Starving your body of carbs can cause tiredness, illness, mood swings, etc. Our bodies need carbs and that's why Atkins goes by 20g carbs a day...it's meant so that people actually do get that every day and not 5g or 10g.
So in my opinion, Atkins has haters because they've seen people who haven't done it properly.