I actually got a fairly decent and quick reply, though the gist of which was that the slogn was because it was only men/boys tshirts and the lack fo female game tshirts was probably one of two things.
1) they didn't think they'd sell
2) the stockists don't make them
Regardless of which is the case I got told that they'd pass on my request to the buying team and that any other female gamers who wanted there to be more gaming tshirts could similarly contact Tesco's to say they would like to see them, as if there was enough demand proven then they might starting looking to stock them.
So if you're a female gamer in the UK, or know of one who might be interested in this, and you want more gaming tshirts at Tescos the email to politely poke them about this matter is: customer.service AT tesco.co.uk