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.