This is a bone of contention for No Kill and shelters. Merritt Clifton has the explanation. May not be what we want to hear but reality often is that way.
The common element may be simply that agencies have insufficiently explained to rescuers how shelter space is allocated.
The trick, for animal control shelter management, is to have adequate capacity to handle whatever crisis comes. This is often complicated because many animal control shelters are too small for their communities––even with more nonprofit humane societies, no-kill sanctuaries, and shelterless rescues helping to house impounded animals than ever before. The total capacity of every dog and cat shelter and rescue in the U.S. today is under 500,000. Total shelter arrivals run close to eight million, 16 times the available cage and run space.