PeerTube uses ActivityPub because this federation protocol is recommended by the W3C and is also used by other projects like the social network <a target="_blank" rel="noopener noreferrer" href="https://joinmastodon.org/">Mastodon</a>.
Remember that PeerTube has only <a href="#who-is-working-on-peertube">one full time developer</a> and a small handful of very involved volunteers. It is not a product developed by a start-up with a full time team and significant financial support.