Because anti-trust jurisprudence in the US has been incredibly poor for the past 30 years. The courts have in general been much more willing to allow industry concentration and will put down anti-trust suits pretty often. It's part of why pretty much every industry has seen a trend towards concentration in the past 30-40 years without much pushback from the federal government and why the last major successful anti-trust action was the Bell breakup in the 80s.

edit: and AT&T only had a 45% market share before the breakup.