From SOFREP:

To put it simply: the one-point sling is not versatile enough. It’s really good at a specific role, and that is accommodating a direct action based use of the weapon. It is extremely good in this capacity but because it is such a specialized piece of kit, it lacks the versatility of the two-point. I am fully aware that they make hybrid slings that can be configured for both set-ups — honestly, this is a good option. But in my mind, the two-point is the appropriate choice when options are limited.