Because it restricts speech and association, on both sides (provider and user.) While minors’ rights in this area are somewhat attenuated, they are not null, and while commercial entities have rights which are attenuated in different ways than minors, the fact that it the regulation is explicitly content based (in that the exceptions are content-based) means the first amendment is more relevant than in the average commercial regulation.

(Depending on how the 5,000,000 user cutoff was reached, it might also have Commerce Clause issues; if its a selectively measure that imposes costs and favors in-state entities by only targeting at a scope that no in-state entity otherwise within its coverage has reached, that looks a lot like a prohibited discrimination against out-of-state business.)