What parts library does mecabricks.com use?

[Orion Pobursky]

ldraw library is free and mature , why mecabricks does not use ldraw?

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.

I might add that I don't really care. I help run LDraw but I'm not biased against other solutions. One day someone is going to come up with the solution that replaces LDraw as the best community run, open source LEGO CAD project. I look forward to that day.