Because, as the FCC is likely to realize, once SpaceX (a US company) is acting as a common carrier for other service provider networks, they will end up being required to carry at reasonable cost and without deprioritization anyone who wishes to do business upon their network with customers, under equitable terms to (in this case) T-Mobile and others — just like the old copper-line DSL networks of the nineties. Space is a limited resource and SpaceX’s exclusive network will ensure a regulatory climate that Elon will loathe, and I expect be compelled under telecom law to accept and obey. I can’t wait to see it.
Happily, anyone could write the FCC and ask what their plans are regarding CLECs and SpaceX today — or, for example, ask questions like “Which US governing body regulates the SpaceX common carrier network?”. https://www.fcc.gov/consumers/guides/filing-informal-complai... is ready and waiting :)