The actual answer is because karyotype testing (testing of biological sex determinations) results in a whole bunch of cis people discovering that they are intersex.

Similarly to why they stopped doing eye color punnit squares in schools, because there are a few more factors in eye color and it was leading to kids thinking they were the result of an affair. Any time there is a test that would uppend the life of the person being tested for no real good reason, we err on the side of not causing a problem if we don't need to.

Should go without saying, but this test should also be included in that, and genital testing should never be a requirement for something as low stakes as sports. We don't test for hyperflexibility, naturally high T levels, or any other genetic advantage someone could have, and any "advantage" a trans person could have (which has never been proven in any scientific study) pales in comparison to the natural advantage some cis people have over other cis people.