If I can recall correctly, the 80's was the doberman, 90's rottweilers, 2000's presa canarios, 2010-today pitbulls. The pitbull breed can have a big litter, hence that's why you can see many of them in the kill shelters, try bringing one to a shelter and most likely they will tell you that they are full of that breed and if you drop the dog it will be put down if no one adopts them.
Another factor why you see many in the shelters is because of the owners, pitbulls are not for first time dog owners. That breed is constantly challenging to be the alpha and the owner/s need to understand all actions from the dog, from having the dog eat last after the humans to making sure when they go for walks they are either on the side or behind or a simple retreat from an owner if the dog jumps on you. if they start conceding the alpha position that's when trouble starts.