[mtrimpe]

Because there are millions of developers who can use the DOM and CSS to create their design and only a handful who can do the same with Canvas.

[possibilistic]

This argument is correct, but you can take it further:

It's not so much about being able to do it with canvas. It's the fact that you have to reinvent all of your tools on top of canvas.

[cia_plant]

When you're tying your square wheels together into a big circle so they turn better, it's time to start reinventing.