The cold hard fact is that there are far more dogs and cats than people willing to take them.
Many shelters now have such high standards for adoption that people are discouraged from adopting pets.
They want to do home visits, get proof of financial ability, past history of adopting or surrendering pets, history of residency...and more.
If you’re not deemed acceptable — and sometimes your current pets too must pass inspection and compatibility tests — the prospective adoptee stays put. Until of course they euthanize him.