Maybe because IBM would have provided a superior price, and RedHat is a public company answerable to shareholders.
Let's take reality, RedHat is still a small player compared to Amazon, Microsoft or Google. They don't have the bandwidth to compete on all the additional hosted services offerings. By partnering with IBM, they get access to IBMs entire suite of enterprise customers and hosted products, making them a serious competitor to the 3 big players instead of being a "me too, cloud". They could make it big together, looking optimistically. But it's on IBM to not screw this up.