Not sure as I am not familiar with other systems. I suspect that a lot of it has to do with age/readiness issues. At least that has come up a lot with all the research piggybacking on the original Van Hiele research. There are lots of proofs in Algebra II as well but students tend to be the same age or a year older depending on whether Algebra II comes before or after Geometry in the math sequence. I have seen proofs presented in Algebra I but generally students are not required to create a formal proof from scratch, just explain the reasoning.
