To somewhat echo BurritoElPastor's comment, running a system/app that can be run in multiple clouds is orders of magnitude more difficult than just running a system/app that can be run in multiple regions.

And, not to be snarky, but many of the other responses that are along the lines of "It's not really that difficult to run in multiple clouds" - let's just say I have trouble believing these commenters have real world experience actually doing this. I'm not saying it's impossible, but it is extremely difficult for any system of reasonable complexity with a dev team of, say, 10 or more people.

And, if you can stomach the cost, you do give up the ability to really use any of the proprietary (and often times awesome) functionality of a particular provider, which can put your dev velocity at a big disadvantage.