Lots of reasons:
* Sellers don't want pictures of their house posted. (Privacy or security concerns)
* Photos scheduled but not yet taken
* Work is still being done on the property
* It's a foreclosure, in which case the banks don't care and neither (in many cases) do the REO agents.
* Agent plans on taking them, but hasn't yet. (That one's not a good excuse.)
* The house looks lousy. (That occurs less often than buyers think.)
Hope that helps.