One of the reasons, in addition to the aerodynamic explanations given above, is that in gusty conditions the flap load relief may operate, 747 varieties. The flaps will retract from 30 to 25 prior to touch down and then in the flare the flaps may well then extend back to 30. Not good when you are trying to sort out the crab.
Anything over 15 kts cross wind I recommend flap 25 and in any gusting conditions over 10 kts.