There are several reasons why you won't find reptiles in a standard rescue organization.
The main reason is that pet rescue organizations are not equipped to handle handle reptiles.
Feeding reptiles means that you have to have a supply of either live insects (which will require care as well), frozen rodents, and or fresh vegetables.
Reptiles require specific temperatures and lights. This can make for a large electric bill for a company on a tight budget.
Most adult reptiles need to be housed separately. The only reason why pet stores can get away with housing reptiles together like they do, is that the reptiles are young and less prone towards aggressive/territorial behaviour. Rescue organizations are typically limited to how much space they have available, and don't have the space for terrariums (or even the money to buy them).
Reptiles require special medical care. Most rescue organizations have vets that will volunteer, or give discounted services, in order to provide the incoming animals with vaccinations, and desexing operations. In order to care for reptiles, there would need to be a specialized vet on hand. Not only are specialized vets more expensive, they're also in higher demand as there are less of them. The exotic vet I go to travels between towns, so I can only make appointments on certain days of the week.
Also consider ailments that reptiles can get from lack of care, such as metabolic bone disease, and rescuing reptiles becomes very expensive and time-consuming.
A secondary reason is that it's easier to market the organization and get volunteers to work at the shelter, if all they're dealing with are fluffy dogs and cats. Even though there are a lot of people who wouldn't be put off by there being reptiles in the shelter, you would have a hard time finding someone who wouldn't be put off by feeding them.
Because of all this, most people will tend to circumvent the process, and handle the adoptions on their own. It also helps if you're rehoming your reptile through a group of people who care about them, you know that you're rehoming your pet to someone who is capable of caring for them properly.
Of course that's not to say that no one abandons reptiles. Snakes, turtles, and iguanas have an unfortunate reputation for being collected by people who want an accessory, and then realize how much work it takes to care for them. Which is why Florida has such a large problem with them, since they're capable of surviving in that climate.
You'll be able to find some specialized rescues in larger cities, where there is a demand for such organizations. You can see a good list of the larger reptiles rescues at the bearded dragon org website. You'll also be able to find some smaller organizations if you're looking around a specific area.
A great many reptiles also get turned in to sanctuaries, where there usually isn't any intention of adopting them into a new home. These are typically reptiles that are suffering from an original lack of care such as metabolic bone disease, or are too aggressive to be handled. These are typically the same sanctuaries that wild animals will be taken to that can't be released into the wild. Although I have heard of some sanctuaries that are specifically for Burmese pythons and iguanas, because of the numbers that are abandoned in their area.