Because that's not their goal, they absolutely don't want iMessage to work on Android, at least not without severe limitations. They want Android to look like a second class citizen. If they bought the intermediary company it would be with the intent of strangling it not expanding it. They'll just slow walk the murder so that regulators don't take too much notice.

For one: it helps them avoid any adjudication that would force them to do just that while avoiding admitting they have the ability to.