Lena seems to have a major role in season 5, yet Jada Pinkett is not in the opening credits. They didn't just stay the same from previous seasons either, as Sinbad's name was removed after he left.

I wouldn't call her major. She was still peripheral. She moved up in the next and last season though.

Probably a financial decision- if the names are in the opening, the actors get paid residuals for the reruns. Pinkett was just as major in season 5 as in season six- only a couple episodes don't have her in them. Most of the seasons of this show have people who were in more than half the episodes but whose names aren't in the main credits:

Year one- Ron and Millie (13 episodes each)
Year three- Ruben Grundy as Freddie's boyfriend in 13 episodes (physically he is in the opening for years 2-5 dancing in some of the background scenes)
Year four- Cory Tyler as Col Taylor's son
Year five- Cory Tyler, Ajai Saunders (Gina) and Jada Pinkett
Year six- Dorian and Tyrell were in 22 episodes and are seen in several scenes in the opening along with Jenifer Lewis. In contrast, Glynn Turman's name is listed and he's seen but he was only in 11 or so episodes

Bottom line is that those younger actors didn't have enough clout at the time to command being placed in the main titles, though one would think Bumper Robinson would have - ADW was like his 4th series.