It was also because there were Republican primary races in play, so Majetter wouldn't have gotten the crossover votes in the primary. I'm not sure she wouldn't have been beated by McKinney, if she had tried to hold her seat.
Let's try that again, and pay attention to the typing!
I'm not sure anything would have held McKinney back. There were Republican primary races in play, so Majette wouldn't have gotten the crossover votes in the primary, that she did in 2002. I think Majette likely would have been beaten by McKinney in the Dem primary, if Majette had tried to hold her House seat. Seeing the writing on the wall, she decided to do the Senate race, which she had to know she would lose, in order to build name recognition for the future.
Either that, or Majette just really let her ego get away wiht her, one of the two.