presumably because of a Pakistan Supreme Court order requiring "[the Pakistan Telecommunication Authority] and other concerned organs of state to take all possible measures for blocking internet web-sites exhibiting the blasphemous caricatures." The source that reports on the blogspot blocking writes:
Update: The block is at the ISP level. Not all internet traffic is routed through the Pakistan Internet Exchange, so the govt. must have ordered local ISP's to block certain websites. All the major ISP's in Pakistan are blocking weblogs hosted at blogspot.com.
I say "presumably" because the sources I cite report as fact only (1) the blocking of blogspot, and (2) the Pakistan Supreme Court order; the remainder, as best I can tell, is still inference, though regrettably plausible inference.