Why can't private buddhas just tell people what they did to achieve enlightenment I mean I don't know if they do but apparently private buddhas don't put forth a teaching, so my question is why can't they just tell people what they did to attain enlightenment.
Because they themselves don't know. It's not like when you become enlightened you automatically understand what happened; the only necessary difference is that you no longer have any craving for anything. Actually understanding how you got there takes a much greater degree of wisdom.

It's much like having an epiphany - you realize something about your life, but you don't likely know what exactly you did to give rise to that realization.