Fearing the loss of the Oral tradition, it sounds reasonable for Rebbi to attempt to compile all known Mishnayos (incl. Beraitot) but he didn't. Actually, it isn't so big and Rebbi with his resources could easily achieve it.
In fact, omitting other Beraytot caused big damages to the Oral tradition - many were completely forgotten, many were made difficult for later generations to comprehend, and more.
I presume Rebbi had a vision and made it on purpose, so why didn't he gather all existing Mishnayos?