The only main reason I can think of, that way people are forced to email them..Maybe they want everything via email?
most people on dp accept pms, only a few chose not to take pms! perhaps because they have got plenty of spams, or may be you are on there ignore list! anyways, i always accept pms, and take a lot of time in reading those, but then shameless promotions in pms i always hate!
I suppose it's because of the increasing amount of PM spam (not just here on DP - every popular forum suffers from it).