The editor "Monaco" was already written... it was for VS Online. They took the already written editor, Electron (Atom Core, etc) from the Atom project (another editor) and grafted the two together. Atom's editor at the time was horribly slow... Electron + Monaco actually worked well.

They've spent the time since then working on actual features... while they'd probably have just barely released anything if they had to do it all in C++, and wouldn't have nearly the depth of extensions they do.