But what did Mark say about those commoner disciples? That they couldn't understand Jesus's message and failed him at every turn, so he taught in cryptic parables to keep his true message a secret.

This was likely because that message, that the world was about to end in divine judgment whereupon the secular winners would be the ultimate losers in hell, was subversive and dangerous. Likewise, that's why so many Christians flock to those compromised Christianities, such as to the prosperity gospel. Commoners don't want to rock the boat but just want to be happy, not to sacrifice their life like Jesus.