No reason other than not wanting to implement and maintain additional wallet connection logic. NWC seems like a good standard; I am sure more wallets will support it over time.
Discussion
deep linking is simpler than NWC though. only maintenance required is adding new wallets, but that doesn't happen often. i don't think any wallets would change their deep link cause that would break things for everyone. you can just copy pasta this if you're using swift for iOS.