system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why AT&T merging with T-Mobile is triggering antitrust challenges but Sirius/XM merging is not an issue.

There are no other players in the satellite radio world so it seems like Sirius/XM have a monopoly on things there.  AT&T/T-Mobile merging still leaves Sprint, Verizon and a bunch of local type carriers (Cricket, etc.) so that doesn't seem to be as big of a deal.
gpt
The argument for the Sirius/XM merger was that it competes with terrestrial radio (AM/FM/HD Radio), thus the two entities combining would not create a monopoly. Also, unlike maintaining the exorbitant cost of cell towers, it's conceivable someone could come along and start their own satellite radio service to compete with SiriusXM by just shooting a couple satellites in the sky.