Princess Kiko's pregnancy

http://news.bbc.co.uk/2/hi/asia-pacific/4688056.stm

The Japanese Royal Family's tradition is that only a male succeed the throne, and until recently, it looked as if that requirement would have to be reassessed. With the announcement of Princess Kiko's pregnancy last week, conservatives and believers in the 'male-only lineage' of monarchs may have one more shot for peaceful male inheritance threatened now by the current all-female line-up.

The conflict over the issue surrounding male-only succession is one that I doubt I can and will ever fully understand.
I suppose just having it as a tradition itself gives it meaning. For example, we don't go switching around the king and queen's roles on a chessboard every other game. We keep queen as the piece as the queen, a very powerful piece, and king as the king, the most valuable piece for capture. We don't change these rules because times have changed--we keep them the same. For that reason, I can understand the desire to keep a male-only rule.
But what is royal blood to begin with? Royal blood probably began with someone or something great that continued to pass those qualities of greatness through from one generation to the next. In the case of the Japanese Royal Family, it supposedly originated from deities themselves. But I wonder what is considered different about a man's ability to pass down royal blood, and a woman's ability? Or is it something about having a woman in power? I suppose a lack of convincing answers to these questions keep me from understanding the issue.
I don't want to have a discussion about feminism, but it is quite interesting to look at the ferocious energy that issues like this can gather.