Because she attacked a sacred cow. Attacking the Obama administration, even if a lame joke, will invite swift punishment.
And it doesn’t matter how loyal one has been up till now. One misstep is enough to get you banished.
Also, ABC wanted to get rid of the show: No pro-Trump statements would be allowed. Perhaps, main reason is that ABC wants to send a message: You speak out against liberals, you will lose your job and all of us (nearly all) will trash you.
Roseanne called VJ an ape, and now you're calling her a cow.
Horrors!