Because sexing requires restraint, and restraint of any kind on a hatchling GTP will cause spinal damage, kinks and other problems such as nerve damage. They are incredibly fragile as babies and shouldn't be handled unless they are on a perch with you holding the perch. Even pulling their tail off of said perch can cause some damage. Sexing the Green Tree requires either probing, popping or the "breeder" lying about the actual sex of the snake. Probing them requires that you restrain them, meaning you have to hold their back end steady with them twisting and coiling, causing unnecessary stress. Popping requires you to press on a very tiny area(in a GTP's case anyway), and that tiny area has very delicate vertebrae that can be hurt by the one doing the popping.

And then there's the lying, but I wouldn't buy from that breeder anyway.

Go email Bob Clark and see if he has any yearling male GTPs. A friend of mine just got a 2 year old male GTP from him, and he is very calm, easy to handle(in the daytime, LoL), and of good size and weight. I would rather buy a healthy,well-established older snake from a very reputable breeder, than a "sexed" hatchling from someone else. Cheaper is not always better.