If a woman's role is really so important, why don't you have a mother to be the Rebbe? Why isn't she the one that you turn to when you really want to know what G-d wants? Why is it that the spiritual leaders in Judaism are the men?

Answer:

Firstly; there have been female "Rebbes" in Jewish history. The Bible tells us of Devorah¹ who judged the Jewish People for 40 years, as well as the prophetess Chuldah² who lived in the times of Jeremiah. The Talmud speaks of Bruriah³, the wife of Rabbi Meir, who had many disciples.

Although this appears to be the exception and not the rule, women have always been, and continue to be, influential and instrumental in their own ways. Men might play a predominant role as public spiritual leaders, but they are not the (sole) spiritual leaders of Judaism. Guidance and inspiration doesn't only come from public figures or through prestigious titles. In every generation and in every community one can find female leaders albeit sans the title rabbi.

Secondly, and perhaps more importantly, the premise of the question is that it would be an honor and "promotion" for a mother to become a rabbi. But is that really so?

Many years before a person ever turns to a rabbi for advice, s/he turns to his/her mother for guidance. Speaking of "leaders in Judaism", it is specifically because the woman's role as spiritual guide is so important, and specifically because she is the one a child turns to when developing the perspectives that will shape his/her life, that any other position a woman maintains, as lofty a pursuit it may seem to be, is a digression from her most noble path: motherhood.