When one looks past the cultural accretions, Mahayana and Theravada are not so different:
https://dhammawiki.com/index.php?title= ... d_Mahayana
The Six Paramitas of the Bodhisattva path are functionally equivalent to the Eightfold Path:
Dāna pāramitā: generosity, the attitude of giving
Śīla pāramitā : virtue, morality, discipline, proper conduct
Kṣānti (kshanti) pāramitā : patience, tolerance, forbearance, acceptance, endurance
Vīrya pāramitā : energy, diligence, vigor, effort
Dhyāna pāramitā : one-pointed concentration, contemplation
Prajñā pāramitā : wisdom, insight
https://en.wikipedia.org/wiki/Buddhist_ ... liberation
High-level Bodhisattvas have already attained Nirvana, yet they voluntarily choose to remain in the world out of compassion for all beings:
As a result of these understandings bodhisattvas manifest meditative wisdom and avoid attachment to either cyclic existence or nirvana.[13] Having overcome all attachments, bodhisattvas on this level can attain nirvana, but because of the force of the mind of awakening they decide to remain in the world in order to benefit other sentient beings.
https://en.wikipedia.org/wiki/Bh%C5%ABmi_(Buddhism)
It would not be surprising if, rather than keeping things esoteric or secret, the Buddha taught Bodhisattva teachings for those interested in the path to Buddhahood, and other teachings for those interested in the personal liberation of an Arahant. Even in the Pali canon, the Buddha taught different things to different people with different temperaments in different circumstances and situations.
The 500 Arahants who participated in the First Buddhist Council would not have compiled the teachings for Bodhisattvas, because that simply wasn't the path they'd chosen. The Bodhisattva teachings, however, were still preserved in either a separate oral lineage or instead in an otherworldly realm.
When the Lotus Sutra teaches that all those who've attained the liberation of an Arahant will ultimately attain full Buddhahood, this shows perfect equality between Bodhisattvas and the Buddha's Arahant disciples, who will ultimately attain Buddhahood too, even if they don't know it yet.
If one has attained the perfect wisdom of an Arahant, this will inevitably lead to also attaining the perfect compassion of a Buddha. It is only a matter of time.