Why? Microsoft is a trillion dollar company. If they want to compete, they can afford to.
If there are 3 roughly comparable companies that control 90% of the market, the competition is already low; if two of them merged, very little competition would remain. A duopoly is not as bad as a monopoly, but is still pretty far from an "efficient market".
But still less competitive than it was. So why allow a company like Microsoft to make the field less competitive when it could afford to compete instead of buying its competitors? I think a company even half as large or profitable as Microsoft should have to present a pretty damn compelling case before being allowed to make any acquisition at all let alone that of one of their competitors.
Your desire to block acquisitions in general is a dangerous one. Doing so, basically kills venture capital markets, and then we get far fewer startups, less innovative disruption and more entrenched market leaders.
Mergers are different because they create fewer giants in the place where more smaller companies used to be. In extreme cases, it may have seriously bad consequences for broader public, and this is a reason to prevent a merger, or even split a monopoly. But this should be a rather extraordinary event.
(Belgium, with all due respect, is not very large.)
But in this case, it will meaningfully reduce competition. For some reason UK's CMA only considered cloud gaming, which is a weird fixation. Maybe because it's an emerging market?
If all Activison Blizzard games become Xbox exclusive will reduce competition and screw consumers, yet that was dropped as a concern for who knows what reason.
It doesn't matter how much revenue Battle.net makes for them. The multitude of developers and publishers that are successful on Steam show that AB is irrelevant to the market. If AB owned a significant number of the top 50, it'd be a different story, but they don't.