I believe the main issue is that Kroger/Albertsons could greatly reduce competition in DEDICATED grocery stores. Sure there's Publix, HEB, and Food Lion, but they're regionally locked while Kroger/Albertsons has a nationwide grasp that can reduce competition in the market of dedicated grocery stores. With this in mind, I believe the FTC has a better chance against them than Microsoft/ABK.
I agree that the Kroger/Albertsons merger is a much more difficult merger under the law. I looked at, and dismissed it, as a possible investment due to the high risk to the deal. The trouble with a grocery store merger is that you are not looking at the national "discount grocery store" market alone. You have all these tiny markets. For example, X-Box and Playstation basically compete everywhere. There are plenty of geographical areas where Kroger and Albertsons are the only two discount grocery stores. If I live in Winchestertonfieldville, Iowa, and the only three grocery stores within twenty miles of my house are Kroger, Albertsons, and Whole Foods, then the merger gives the new Kroger/Albertsons merged entity a monopoly in my local discount grocery market. All they have to do is charge slightly less than Whole Foods, a premium grocery store, and they are still the discounted entity. They therefore have the ability to unilaterally raise prices without losing customers.
Divestiture in the discount grocery market has not been successful in creating viable competition in the past. It has been tried. The problem is, if you spin off stores in the few areas where it is necessary in order to keep competition alive, then those spun off stores now lack access to the scale of supply chain necessary to effectively and profitably run a discount grocery store. They either have to raise prices, or they go out of business. Either one substantially reduces competition in that market.
Antitrust concerns can be very problematic in things where you are looking at local markets, particularly when administrative agencies are inclined to look very carefully at the viability of divested assets as a going concern. This iteration of the FTC seems likely to be interested in looking at that, and it would seem to not bode well for that merger. I will also point out that the FTC is accustomed to dealing with examining individual local markets, and it is something they are pretty good at. They examine hospital mergers regularly to make sure that one hospital in a region is not acquiring their only viable competition for any subset of customers.
MSFT/ATVI is a much more difficult case for the FTC. This is primarily because anyone who competes with them anywhere can compete with them pretty much everywhere. (Geographically that is.) That gives them a lot of less options for how to define markets. They cannot limit it regionally, except to the US market, and are instead forced to differentiate only along the lines of product offerings. When you have the FTC run by people that are anti-merger generally, or anti-merger in your particular field, less tools in their tool chest is always a useful thing. Even when they have the right tools, they don't always know when to use a hammer, when to use a screwdriver, and when to use a socket wrench. Accordingly, they have let some grocery mergers through that, in retrospect, seem to have been a mistake. But now they've tried selective divestiture and seen it fail. That's why I personally did not want to bet on them making the same mistake again.