Why is it so hard to adopt from shelters?

My mom went to the shelter here where I live. She was wanting to adopt 2 white kittens that were sister and brother that we found on petfinder. Well, she went up there to try and get them and they told her she was unfit because her cats rabies were 2 weeks outdated. She asked them if she got her cats rabies the next day if she could come back and get them and they told her no because the cat should have already been taken care of. If they want to find these animals homes so bad then why in the heck are they so strict? They should be glad that somebody wants to save 2 lifes. In my opinion with it being so hard to adopt that is why some people choose to go to a backyard breeder! That is just my opinion. What do you all think?