why does louisa agree to marry mr bounderby?

Louisa consents to marriage with Bounderby because her brother Tom encourages it. Tom tells Louisa that if she marries Bounderby they'll be living under the same roof again, and that through their marriage he might gain a bit more freedom from Bounderby's authority. In essence, this is just another decision she makes based upon the love and protection she feels for her brother.