Pretty sure comcast is legally allowed to block CNBC or Fox.

Cable channels regularly get dropped from lineups due to disputes over fees. Here’s a recent example of NBC in threat of being dropped. https://www.pressherald.com/2019/10/02/spectrum-may-drop-nbc... Edit: that link has a paywall, but here’s another discussing cable channels being dropped https://www.post-gazette.com/ae/tv-radio/2016/02/14/TV-Q-A-D...

There’s no general right to force cable providers to have your cable channel.

Comcast pays to carry CNBC and Fox content. They can freely decline to pay the content providers what they wish and drop their content. It is not an uncommon occurrence.

Fox and NBC cannot provide air time for one political campaign and deny air time for an opposing political campaign. Twitter is more like Fox and NBC rather than Comcast.

Twitter is nothing like Fox and NBC, unless they've managed to pick up a broadcast license without anyone noticing.

Fox and NBC are only required to provide political campaigns equal access because of their licenses to exclusive use of portions of the public spectrum.

Cable channels like CNN and Fox News don't use this spectrum, and thus don't have the same requirement. Twitter is more like a cable channel than a broadcast station.