Because people aquire animals that they are incabable of dealing with as an adult!
I hear it all the time at shows. They think they will keep it for a while and when it gets bigger they can just sell it for a lot of money or just give it to the Zoo.
Wrong on both ideas!
Next thing they know they have a monster on their hands that eats them out of house and home and is a potentially dangerous animal.
The rescues and shelters are full and they discover nobody wants it and they assume that since the glades are similar to the native habitat that it is the best thing to do for the snake and to get THEMSELVES off the hook.
So they take the easy way out.