I couldn’t give a rats about FLAP or FLAPS (although Airbus want the plural as per the FCOM), but I do care about Zero vs UP.
You call the position of the Flap, 0,1,2,3 Full.
Gear is binary, DOWN or UP.
Also, to mind there’s less chance of confusion or mishearing. The word UP or DOWN can only be attributed to one action.
That said I acknowledge that Flaps UP has been around for years in the B camp. Not saying it’s wrong, just that it can only be better having a verb associated with one item. After all the flap lever is a flap shape and the gear lever a wheel shape for a reason.