Because it turns out, Mom just isn't funny. The cast is capable, but they really have nothing to work with. And I would forgive the show for being something other that humorous, if they were delivering on the narrative end. I'll give you two guesses on how they're doing there.
This week's episode was preoccupied with Anna Faris's daughter, Ms. Stupid Face, finding out she's pregnant and deciding to keep the baby. Here both the narrative and potential for off-color humor are both sacrificed for the politically correct demonstration of how a family "should" react to the situation. Anna Faris and Allison Janney both politely await Stupid Face's decision, which takes her a total of half an hour after the pregnancy test, and when she announces that she's keeping the baby, they just hug her and say "I support you."
I don't care where you fall when it comes to the social issues and choices for teenage mothers, I think you can all agree that the described scenario is just atrocious writing. There isn't a single solitary parent with pretenses toward loving their kid that is going to restrain offering up their opinion, making the girl hash out their options with them as a sounding board, and essentially be involved with a high school student's decision to raise a child, most likely in said parent's house.
This isn't made any better when the boyfriend, an unlikable version of Modern Family's Dylan, comes on the scene and takes five minutes to figure out that if his girlfriend is pregnant, that means the baby is his. Again, zero reaction, contribution, or insertion of either mother or grandmother. They respect the daughter, who so far has existed only as a sulky, entitled blob in the show, far too much to it infringe on her rights to her body.
This is both unrealistic and by extension uninteresting. No one wants to see the perfect scenario for teenage mother's rights as it would exist in a vacuum. People are far more complicated in their interactions with each other.