SNL's rating keep going down because the writing is sophomoric, the skits are mean-spirited and cliched instead of clever or funny, and people are simply worn out by all the nasty, rude and hyperbolic political impersonations and sketches.
The pacing is awful, too, and the "stars" seem to have the attitude that just their showing up is a reason we should watch the show.
What started out as a clever, entertaining, really fun show with skilled comedians and great writers has now devolved into a rude and disgraceful waste of time.