I like plain yogurt. Why can't I use that instead of your recommended greek yogurt?

You may absolutely use any yogurt that is, single digits of sugar. I, personally, like the 100% protein oikos which has 15 to 20 grams of pure protein and 0 grams of sugar. But again, the key is, keep your sugar low, your protein high. and you can have any yogurt that you enjoy that fits that parameter.