I suspect that what you really mean is 'why won't my dive shop fill my 300 bar cylinders with nitrox?' And the answer is probably that they either haven't got the kit, haven't got the time, or can't be bothered.
Adrian is right that you won't actually get a true 300 bar fill, but mixing nitrox to higher pressure requires a bit more care and time. I've certainly had 300 bar cylinders filled to 300 bar and more with various nitrox mixes but I've had to ask very nicely and give the shop a couple of days to fill them in their own time.
<font color='#F52887'>Hi Robert,
What I meant was, I thought that once you filled nitrox to 300bar the pressure meant that the mix was unreliable to get right due to the differing properties of different gases to pressure above and beyond around 200bar. I suppose that the answer "shops wont do it because it is a pain in the arse to get right" answers my question. I knew that I should have listened at school!