For one, it would give the resulting entity a majority stake in Hulu and control over a lot of content that they could potentially yank from the likes of Netflix. It's also the default position of regulators that horizontal mergers are bad for competition, so I'd expect a fight regardless of the ultimate basis [1].
I think the argument that content itself comprises a monopoly is a tough sell since content is something that can be demonstrably created by lots of parties (Netflix, Disney, Amazon, Apple, NBC, YouTube, etc.). In fact in today's environment it would be nearly impossible to form a monopoly because content creation and distribution has been so democratized.
If they merged with Netflix and controlled pretty much all distribution, then that might be a more compelling argument, but still difficult.
It seems to me that people throw the term monopoly around way too freely. Two major content distributors in today's environment is not destroying Netflix's ability to compete. When Microsoft controlled the OS of just about every desktop in the world and wouldn't allow competition in the browser space, that was a monopoly. When Standard Oil owned every oil field in the United States and the gas stations, that was a monopoly.
Is the majority stake in Hulu really an antitrust concern? Streaming services are pretty competitive and Netflix has plenty of their own exclusive content.
The content ownership example that I provided is absolutely not enough to merit blocking the merger on its own, just one example of consolidated power that will likely come up in the event of a hearing.
What specifically about this deal should be of concern for antitrust regulators?