"Why are left foot walks forward in tango are always in CBMP?"

The short answer is "because that's the character they decided tango should have".

The long answer is that this is the way tango avoids bumping legs with your partner. Instead of stepping right at her, you step almost behind her, very nearly your left foot under your right wrist.

"E.g. first step in Basic Reverse turn or Open Reverse Turn?"

Well actually those have the CBMP of any left foot tango walk, but also CBM because they are initiating a turn.