Boxers, Beagles and Pit Bulls are some of the most common breeds in America. you might see them more often in shelters simply because there are more of them out there in general.
The other thing is that with popular breeds, people tend to get the dog because it is popular, or cute, without seeing how it fits into their family. boxers and Pits are both high energy dogs. They can also be very destructive if their energy isn't properly channeled. Beagles can be very hard to train- its part of their hound nature. Unfortunately far too many people buy them without knowing any of that. They just know that a beagle is a dog, and they want a dog.
The other thing is that the more popular a dog gets, the more poorly bred specimens are available. Labs, for instance, have a reputation for being great family dogs. Most labs are. However, when a breed gets very popular like Labs, you see people breeding just for money, without any concern as to the integrity of the breed. People who don't know any better then buy these dogs, and have a whole host of issues. They may have expensive health issues, they might be stubborn,they might have insatiable energy, or the size of a horse etc. None of which are supposed to be Lab traits, but which you see all too often from indiscriminate breeding. People get them after reading labs are easy to train, gentle, mild mannered and fairly healthy, but the dog they got is NOTHING like what the breed is supposed to be like. Someone who may have done well with a well bred lab, may not be equipped to deal with a stubborn, high energy, 110 pound monster.
The final issue is adoptability. Pitbulls are hard to adopt out. Meaning that they sit in shelters longer. A shelter might get in 20 labs in the same time it took to get those 10 pits, but the labs are easily adopted. So you see a few labs in the shelter because none stay for long, and 10 pit bulls. In my area, for instance, small dogs get adopted almost immediately. Its not that they never get surrendered, just that they get adopted very quickly. Often the day one becomes available, there will be a line of people waiting for it before the shelter even opens. Pitbulls are hard to adopt out because they have a negative reputation. They are also not allowed in many apartments, you can lose your homeowners insurance for owning one on some plans etc. So even people who DO want them may not be able to have them. many people cannot distinguish a pitbull from a Boxer, which can also make Boxers harder to adopt out. Actually my local shelter intentionally labels Pit Bulls as "boxer mixes" to try to move them out the door (not a move I agree with, and I love pit bulls). However, people see a boxer, think its a pit, and then don't want it. This isn't just a pit bull issue- in my area Rottweilers, Dobermans, and Chows also have bad reputations, making them harder to rehome.