The reason could because of how these payments apps evolved.
People already had cheap android phones in their hand. After the 4g boom, many got their first smartphone.
UPI related playment apps started with QR code scanning, just like in China.
There is no extra cost to the vendor. All they need is a sticker with their QR, which customers can scan. Later, they came up with a (bluetooth) speaker which can announce payments. It doesn't even need a connected smartphone.
If they used cards, they need to manufacture it and the readers. Ship them to their home addresses (which is complicated and lossy), train them about card usage.
For many vendors who accepted UPI payments, it was their first time accepting something other than cash as payments.