User blog comment:FairyRaven/Fairy Tail Chapter 437: Magnolia, Review and Commentary/@comment-24277053-20150525114500/@comment-7418318-20150526070848

Who said she has to be married and a mother to become Master?

I just think it'd be nice for her to have a few more comparetively carefree years as a young wife and mother before taking on the heavy mantle of leadership and the responsibilities, duties, worries and stress that come with the role.

Beside that, being guild master would probably mean she can't just go on adventures with the team, so I expect that if she becomes Master in teh story, it'll only be in last few chapters, probably the epilogue.