They can get more speed from a narrower frequency band with 3.1 vs. 3.0
This is why cable companies are upgrading to the 3.1 standard.
It isn't about the fact that 3.0 is capable of those speeds, it is about getting those speeds within the bandwidth the cable company has to work with on their system. 3.1 allows them to do more than 3.0 can.

I have only spec'd 3.1 modems for my customers for awhile now. Xfinity has started offering gigabit service in my area now and 3.1 is required. So my customers are already ready for it, at least the ones I've installed within the last year. They won't have to buy a new modem.

Honestly if you are selling cable modems to customers, you should have been selling them DOCSIS 3.1 for a long time now (at least the last year or two). They are backward compatible to 3.0 and everyone has known for a long time now (many years) that 3.1 was going to be required at some point.
Shame on those who haven't.

I'm currently using ARRIS SURFboard SB8200 modem, available on Amazon for $149.