As far as I remember shareJS was available when we started to work on tree-based OT. We researched it but it turned out to be too simple for what we wanted to achieve and there was no special handling for some edge cases that happens during collaborative editing.

We achieved something similar to shareJS functionality quite early, to be honest. What proved to be extremely difficult was all the extras to smooth up the user experience. We would have to (heavily) extend shareJS anyway, so it made more sense to go out with own solution, crafted for our needs.