IBM was profitable but stagnating, they were already a major reseller of Red Hat, who were growing rapidly, had good tech, lots of potential to grow more, and was already big enough that they may be able to change IBMs numbers by enough to not just get lost in the noise (if you make $8.5+ billion profit changing that by $100million isn't going to get wall street excited)

Someone like MS would buy Canonical for completely different reasons, they're not big enough to effect Microsofts profits in any way, they dont have any really interesting tech, but they do have the Ubuntu brand and control of one of the really big linux distros (the biggest on the desktop) so there might be value in Azure being able to brand itself as the home of Ubuntu ("it runs perfectly on our cloud becuase we write it that way"). To be honest though I dont see that as a big enough thing to make MS risk their reputation as a good place to run Red Hat and openshift. The only other reason to buy Ubuntu would be to kill it, and the political ramifications of that to the open source friendly reputation they've worked hard to build would be bad.