Well, that is because the Influenced status has a lower progression step value than the Member status, and system doesn’t allow a person (or more appropriately program member) to be moved to a status with lower progression step value from a status with the higher progression step value. You can modify the channel status’ step values so you can move people to the status you want by going to Marketo Admin > Tags > Channels. You can Use the equal values when statuses are intended to switch back and forth as opposed to a progression. Read all about creating/updating a channel here.
However, technically from Marketo’s point of view, influenced status means that a person has just been influenced from the marketing initiative, whereas, member status means that the person has reached the success step/has engaged enough with the program to meet the threshold requirement of the program initially set (e.g., filled out form). That’s why Member status has higher progression step value than the Influenced status value and you can’t move people with the Member status to Influenced status.