From my experience, it has been the discretion of the FA or the GA.

When boarding full flights, FAs have told me to pre-board them in order to make sure they put away any carry-on items. PAX in row 1 do not have the luxury of space in front or above. Those in row two have very less if those in row 1 take it all.

The over heard bins in front are usually closed during boarding so those in back do not take up all of the space up front.

When the flight is not so full, then this is not necessary, and so are not usually included in pre-boarding.

BTW, they probably don't want to have to deal with PAX complaining that they do not have space for their bags, especially women with large purses (no offense) that need to stow the bags.