At the time we started revamping our editing interface many of the options were still in early/incomplete places. I think ProseMirror had an funding campaign at the time that didn't meet its goal initially, although did later on in a second attempt. Ultimately we decided to venture down the path of building our own editor, which was based on a combination of our own learnings in text editing and a collaborative system some folks in our graphics department had put together.

About 6 months in, near the start of 2016, when moving on from the exploration/demo stage of the project to actually building out the real deal we took another look at the landscape and decided to move to ProseMirror. I don't remember the exact points that went into the decision but transitioning from our in house solution to ProseMirror felt pretty natural as they followed similar design philosophies.