It wasn't, they genuinely believe in Beeper, and the long term plan was rather to become the leading chat app in their own right, with users communicating over the Beeper protocol.
A bit of pedanticism here - it's not the Beeper protocol, it's the Matrix protocol. Beeper is built on top of Matrix (using bridges that you could host on your own server if you wanted).
What i meant was that the idea was that people would eventually stop using e.g. whatsapp on Beeper and just use Beeper, since eventually all their friends would also have the beeper app installed. Might sound a bit optimistic but there it is.
Except there are a lot of Beeper-only Matrix spec implementations that aren't available for everyone else. In other words, no other Matrix homeserver or client can benefit from the features Beeper has.