It’s hard to block mergers based on a company involved being a monopoly if none of the companies involved are monopolies or will become monopolies.
Regulators have to come up with a different set of rules to block “large but not monopolistic mergers” without also just effectively protecting the actual leader in a given industry from competition.
Being the market leader, Sony will have a much harder time making larger acquisitions than MS did, and this ABV merger didn’t exactly breeze through.