People react in different ways to people of different race and gender - there are story possibilities. That's the reason to do anything in Doctor Who - because there's a story in it.
One of the problems Doctor Who has is that due to its structure - educated middle-aged man accompanied by audience-surrogate young woman - it's effectively Pygmalion in space. It's an inherently sexist programme. The production team can adjust it a bit, they can work on making the companion more of an equal (which has only been properly done once with Romana II), but the structure is always there. Having the Doctor be a woman with a young male companion for a few years would be a refreshing break out of the box in that regard.
But the bottom line? Because there's a story there.
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One of the problems Doctor Who has is that due to its structure - educated middle-aged man accompanied by audience-surrogate young woman - it's effectively Pygmalion in space. It's an inherently sexist programme. The production team can adjust it a bit, they can work on making the companion more of an equal (which has only been properly done once with Romana II), but the structure is always there. Having the Doctor be a woman with a young male companion for a few years would be a refreshing break out of the box in that regard.
But the bottom line? Because there's a story there.