If you're assuming "mainstream" Wing Chun is what WSL was working with from YM and he went about overhauling it by himself, changing almost everything about it save some superficial similarities into a far more practical and coherent system, that's not a simple explanation at all! That would be like generations of innovations performed by one guy. Hence, I said it's incredibly unlikely if not impossible. You may not see it that way, but you've not experienced WSL's system to see it from that angle.
The simplest explanation that I can see is just that YM wasn't very concerned with cranking out numerous high-level students, didn't have the patience or interest in teaching those less serious or talented, and indeed taught very few people the system. May cause butthurt, but that's the truly simplest explanation.
The simplest explanation that I can see is just that YM wasn't very concerned with cranking out numerous high-level students, didn't have the patience or interest in teaching those less serious or talented, and indeed taught very few people the system. May cause butthurt, but that's the truly simplest explanation.