Because the SBLM is a completely outdated test and many testers, even gifted specialists who have long loved and advocated for the SBLM have stopped using it. While there is a possibly a valid argument for using the SBLM in addition to a currently accepted test I would definitely not be happy using it on it's own.
We had our DD tested with the WPPSI at 4y9m and she had multiple ceiling issues. She was retested at 5y3m on the SB5, which was a much better test for her (and she was much better off with a gifted specialist tester)
