[yummyfajitas]

What possible reason would the Lego corporation have for discouraging 50% of their potential market from buying their products?

[uiri]

By discouraging one 50%, they are encouraging the other 50%. If the increase by encouraging the latter 50% offset the decrease by discouraging the former 50%, they get a net increase in profits. It is a lot easier to market to either boys or girls than to market to boys and girls, so marketing to one half or the other tends to be more profitable than marketing to both.

[rue]

A possible reason is that they believe they'd be less successful if they actively tried to broaden their consumer base.

Whether that's correct, I couldn't say, but it's certainly not an impossible scenario.