C-SPAN. Both DBS companies are obligated to carry a certain %age of "public interest" channels as a condition of their licenses per the FCC. However, the rule stipulates that they cannot carry more than 2 from the same provider. Thus they carry CSPAN 1 and 2, but not 3, because if they carried 3 not only would it not count toward the "public interest" condition, 1 and 2 would not either. Which means adding 3 would require adding not only it but 2 other PI channels.
CSPAN is aware of this and has asked the FCC to waive the rule, but this has not happened. But it allows anyone with any cable or sat. subscription to watch all 3 on the internet.