I've found that there are a few typical reasons:
-It's not always clear to PM's whose decision something like that is. Whether it's a grounds expense, maintenance expense, a tenant expense, or a general expense. Each different department is going to say that it's another department, because the employees get their bonuses by keeping their department profitable.
-Most PM's own a washer (not that they know how to use it), but they find it hard to justify the expense of paying out when their guys could do it for $12/hr.
-Most often, though, I think they just have no idea what a difference it can make. Most of the commercial referrals I get are from residential customers who understand the value of a good clean.