"We have nowhere else to go and Amazon know it." The FTC and states are going after Amazon, largely for stopping merchants from offering discounts elsewhere.
Did they do the same thing in the UK where they were also being held up? No. Was the trump admin particularly hostile to this or the Biden admin particularly lax on it? No. So it has nothing to do with the admin, it's just they have hoards of lawyers that can exploit loopholes or make good defences.
Is it the FTC's fault at all? Lina Khan is doing great work, and the US has fought against Microsoft's monopoly before. The real problem is the judicial system has no interest in joining this fight.