I did some research before I started and read about Bespin. I don't think they merged into Ace because of the Canvas. It was probably because another team on the same company also worked on an editor, witch had the same goals, and aimed for the same market.
Careful! Mozilla tried that as far back as early 2009 (the project was called Bespin, you can google for "mozilla bespin canvas" to find out more).
The canvas-based approach was eventually abandoned and the project merged into Ace, a more traditional DOM-based editor.