Because we need a canvas-blocker add-on and a flash-blocker
And the problem is these "canvas-blocker's" don't exist yet. For nearly a decade, I was perfectly happy with the speed of my browser when I had flash set to click-to-play + an ad blocker. But all the flash haters got there way and "killed" flashed.
Congratulations! You replaced it with an inferior standard that's now built into HTML5, so it's no longer trivial to block. Good job people! Flash is back, more CPU demanding than ever, and even harder to block.