The Variety article Yvette Nicole Brown Leaves ‘Community’ cited personal reasons:
Yvette said she is leaving the show for personal reasons: namely, to
take care of her ailing father.
“My dad needs daily care and he needs me,” Brown told TV Guide. “The
idea of being away 16 hours a day for five months, I couldn’t do it.
It was a difficult decision for me to make, but I had to choose my
dad.”
Shortly after leaving Community she was promoted to series regular on CBS' Odd Couple reboot. According to TV Line Brown highlighted the differing show formats as the reason for her depature:
As Brown explained earlier this week, the multi-cam Odd Couple —
versus the single-cam Community — is “a better fit for the life I have
now,” adding, “My dad needs daily care and he needs me. The idea of
being away 16 hours a day for five months, I couldn’t do it. It was a
difficult decision for me to make, but I had to choose my dad.”
Given the good terms on which she left Community the door is probably still open for her to make a guest appearance, but I couldn't find a source confirming that.