I think Broadcast Box may have implemented WHIP/WHEP before MediaMTX so it tends to be one of the first results when looking up a WHIP-capable server. Plus it has a public instance so if you want to test WHIP real quick, it's pretty easy to just point OBS somewhere instead.

MediaMTX used to be "rtsp-simple-server" which really undersold what it was capable of (even back then it could ingest RTMP and output HLS and WebRTC).

Overall I think MediaMTX has more features and can do everything that Broadcast Box can.