Canvas is less of a piece of drawing software and more of brainstorm/mindmapping/idea workbench software. Excalidraw will satisfy different use case than Canvas and you can definitely use both!
We considered many options to use Markdown but came to the conclusion that Canvas is not something that can fit into a readable Markdown file. Either the Markdown file would so messy that it becomes pointless (i.e. you would never open it in Typora to edit), or it would severely limit the power of Canvas.
After much internal debate we chose the JSON format. We stay committed to keep it as open and easy to work with as possible. Plugin developers are already parsing and modifying the JSON file to programmatically change a Canvas view, and I think that's a fantastic start!
https://obsidian.md/canvas#protips