AFAIK it's a matter of scale. Some if not all of the parts needed can only be bought in multiples of 500 so to make one you would have to make 500 minimum. Even at a cost of £50 per unit (which is a conservative estimate) that's 25k before you have even factored in manufacturing costs and everything.

I believe there was talk of licensing the production out for free but even then it's a matter of finding someone willing to put their money where their mouth is and I guess there's nobody out there with big enough balls...

Anyway, I'm sure someone in the know could give you a more informed answer, I'm just going on what I've gleaned from this forum!