Because advertising your lack of girls is not viewed by the average woman as a hopeful sign. (Heck, I'd think twice about any online site that advertised itself with "we need more boys".)
Also, the above point should be sufficiently obvious that a potential female reader would look at that and justifiably think "This person is thinking about what they want and not thinking about how I might react" which isn't much of a hopeful sign either.
I'm probably non-average, but I'm ambivalent about hearing "we need more girls" from any community that's generally interesting. The first question that I think of is "why don't they have any?", but as long as it's not obvious to me why there are not presently enough girls had by a website and it's easy to leave if I find a compelling reason later, my obliging nature would be likely to take over. Also, saying "we need more girls" does advertise the lack of girls - but it also advertises the recognition that maybe that's not ...
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