I honestly believe it's because Madge Sinclair the voice actress of sarabi died in 1995, and they didn't want to write her character out per say it's just up to us as the audience to assume she's in the back ground. It's kind of dumb seeing as that we would definitely notice she was missing. Although I really like the sequel unfortunately it was very poorly made. :-/
I've always thought and believed, and still do, that it's as simple as this:
Because Sarabi wasn't in any way essential to anything in the story they wanted to tell. As it wasn't about Kiara learning to be a future queen. But it was all about Simba and Kovu following in their fathes' footsteps (in Kovu's case adoptive father's) and about Kovu and Kiara's love story. If Simba needed guidence as a king, he would've asked the Great Kings of the Past which he attempted at one point. Sarabi simply wasn't needed so she was left totally in the background, up to the viewer's decision as for her situation.
Sure it would've been nice if they'd shown her even for a few seconds without any sopeken words, somehow interracting with Simba...But as the movie worked without it and they understandably wanted to focus on essentials...