How is it impossible to use? Apparently everyone in India is using it? Apparently only tourists face issues with it?

With visa and Mastercard you pay transaction fees and rent for a card terminal aka card reader, which are lower or non existent with these homegrown or local P2P payment systems. Outside western developed countries aka the US and the richest European countries like France, Germany, that's enough to stop using visa and Mastercard.

Impossible for tourists to use yes. You need an Indian bank account and there are services ostensibly for tourists but they still don't work everywhere, for example many vendors will want friend payments instead of business payments because they don't want to pay the surcharge.

Tourist UPI only permits P2M payments, and most normal transactions (as well as refunds) are conducted P2P.