Basically, because they just burned the lulav that was hanging over the door and are afraid of jumbi and/or they feel uncomfortable when the house is too clean. By the time succos rolls around again the mice will have eaten the matzah, so they hang a new lulav. It seems more jewish to use matzah and lulavs than holy water and crosses.
P.S. this has been discussed in the CR before. Good things come to those who search.