We originally chose keysend because you can send metadata (sender name, total amount before splits, message, podcast, episode, app name, etc) along with payments through the Lightning network. You didn't need any web server or service sitting in the middle.

Keysend also just lets you send a payment without requiring the other side to generate an invoice first. Most of our feeds have payment splits in them for the podcast hosts, crew, and anyone responsible for putting together a podcast and it's a bit cumbersome to have to generate multiple invoices for every single split.

We've since updated the spec to allow for a Lightning Address to be used in the feed which allows the app to use either keysend or LNURLp depending on what the sending and receiving wallets support.