This is somewhat of a tough question to answer, because there not merely one single reason why Sidi does not choose to marry Lakunle. Her reasons for not marrying him also probably change over the course of the play. Finally, the answer is somewhat subjective, because Sidi probably doesn't reveal all of her reasons for not marrying Lakunle.

Sidi is a beautiful woman, and she knows it. We see Lakunle making his marital intentions clear at the beginning of the play, and we can't help but think that Lakunle is not the only male in the village that does this. This thought is later confirmed when Baroka makes it known that he wants to marry Sidi.

Sidi repeatedly refuses Lakunle because he won't pay the bride price. This is a significant reason why she refuses to marry him. The bride price was a tradition of the villagers, and Sidi wants to honor that custom. She refuses to marry Lakunle because he won't pay the money; however, there could be more to it.

It's possible that Sidi simply doesn't like Lakunle and that the bride price is a convenient way to say no. Lakunle is not likable. He comes across as arrogant and tries to convince people that this is merely because he is modern and forward-thinking and everybody else is stuck honoring backwards traditions.

As we get to know Sidi, we realize that she is power hungry, and she begins exerting her false power once the images of her in the magazine arrive. It's clear that wealth, status, power, etc. are important to her, and this could be another reason why she doesn't want to marry Lakunle. He is a lowly schoolteacher. While the position does require education, it doesn't come with a lot of money or societal power; therefore, it's possible that Sidi doesn't marry Lakunle because he won't give her any upward mobility and status increase in the society. This could be why she finally marries Baroka: he already is the top man in the village..