(2020-09-11, 14:47)Orion Pobursky Wrote: I don't know. I'm sure the reason is something like this though: "I started my project and didn't know about LDraw. Once I learned about LDraw it was easier to write a converter to convert a model into my format than it was to recode my entire rendering engine" or "I was dissatisfied with how LDraw handled <issue> so I wrote my own LEGO CAD program to not include that <issue>". Both of those reasons are by far the most common for why a particular project doesn't natively use LDraw.
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I believe (though I can't find the information at the moment) that "Scrubs" (the author of Mecabricks) know about LDraw but wanted to more accurately model the bricks (e.g. part measurements in fractions of mm) than that offered by the LDraw standard. For most of us (me included), LDU's are a sufficiently good enough basis to use when model things. Scrubs wanted greater accuracy. If I manage to find the article where I read this, I will post a link.
Regards,
David