If they did legalize it, what would happen to the thousands sitting in jail / probation for pot offenses. Oh were sry, it's ok now?
It's also politically hard enough to make people approve full MJ legalization, without having to fight the ghost of a wide-scale amnesty that would likely threaten deeper social repercussions.
In practice, a sort of low-scale amnesty for smaller infractions would likely have widespread support, even just to clean up personal records.