In my opinion, the reason this seems to happen especially in this way, is because unfortunately most people don't give reptiles the same care as other mainstream pets like cats and dogs.
Also, new reptile owners that buy on impulse often get dissapointed that their new pet isn't cuddly and doesn't interact much. You can't train it to do any cute tricks and it won't come to you if you call its name. These animals just don't show emotions and that is a big let down to newbies. They often think "I'll care for mine so well and handle it often and it'll become tame, want to be with me, etc..
Once the novelty of having an exotic pet has worn off and that the emotional bond never materialized, they lose interest and care for the animal tapers off often to the point of neglect. Then they just get rid of it...
On the vet care aspect, many don't accord their reptile pets the same medical care as their fuzzy companions. Why? I suspect that the lack of an emotional bond is the big factor. When something's not right with your pooch or kitty, it shows it and because this is a pet that your emotionally attached to, can't bear to see in pain or sick.
On the other hand, reptiles don't have that mutual emotional relationship, it's one way and only for some. Not all keepers truly love their reptile pets, for some it's show or coolness and God knows whatever reason. Also reptiles don't express health problems in the same way; they don't wimper, give you puppy eyes, limp on one leg, etc. A keeper has to keep an eye on different aspects to ensure good health.
At my vets clinic, I am one of only a handful of owners that he has seen in the past couple years. He was more than shocked to see me come in with a dozen herps for a basic checkup and fecals! Never happened before. I see him because although he doesn't treat many reptiles, he is one of the only ones in town that does and he is quite good. If he's not sure about something he will research and confer with other vets to be sure he takes the right course of action. He has also had a few herps as pets too so I know he loves them, a big plus for me.
Keeper of 5 snakes, leopard geckos, 1 green iguana, 20+ tarantulas, 2 dogs & a bunch of rats!