Why? There are already all kinds of web sites that do this kind of thing. Monodraw's unique selling point is that it's a native Mac app that takes advantage of the Mac UI and it's done well so the UX is top notch.
If you don't care about making the best possible app that you can, go ahead and do it in the browser. You will get something that's probably good enough and runs everywhere. But it's going to use more battery, more memory, and more bandwidth and not feel like a Mac app. Plus (IMHO) it's less fun to develop for the browser.
I believe the attention to detail that sets Monodraw apart can be transposed to the web as well — albeit diverging from MacOS conventions.
It’s possible to make great web apps, it just takes the kind of care and dedication that @milen has already proven to have. If the web interface lowers the barriers to developing a cross-platform version of Monodraw, then I think it would be silly not to consider investing in it.
If you don't care about making the best possible app that you can, go ahead and do it in the browser. You will get something that's probably good enough and runs everywhere. But it's going to use more battery, more memory, and more bandwidth and not feel like a Mac app. Plus (IMHO) it's less fun to develop for the browser.