Small businesses generally dont because the setup for a merchant POS is quite a bit of hassle. In fact maintaining that device also costs money. A UPI merchant on the other hand is quite easy and free from what I know. This setup with visa mastercard might change in the future (with square like devices). However there is also a security problem so fraud might increase with that. Very interesting times for visa and mastercard who have basically not innovated since their existence and yet somehow have grown so big.