I suspect shelters are disproportionately full of pit bull mixes that are difficult to place.
A few years ago I looked into adopting a shelter dog and all of them for a 50 mile radius were pitbull or pit mixes.
Your suspicion is correct. I’ve looked for dogs twice this year, and viewed many many shelters online. A solid 60% at each and every place are pit or pit mixes. Another 20% are Chihuahas, German Shepherds, or Huskies. I’m absolutely not kidding “All other” are only about 20%