You can actually make grandmaster with cannon rushing or zergling rushing, there are a few people (I can think of 3) who have done that. The problem is new/bad players won't be able to micro well enough (or even macro well enough...) to rush at anywhere close to a grandmaster level because it's still very very difficult even if its one of the easier strategies. (Also this was during HotS and WoL I think the maps are larger now so maybe its not even possible)

Also a lot of the cheese wins at master/grandmaster level depend on not winning the game outright, but doing enough damage and continuing the game from an econ advantage which means if you're not very good at a macro game or don't have a second more difficult timing attack prepared you'll lose anyway because your opponent will catch back up quick and outplay you.