I was told, when I rang up the blood donation people to ask why I couldn't donate, that it was because I had type 1 diabetes (nothing to do with being on insulin), and so there would be fewer platelets in the blood. So they would have to take more blood to get the same amount of platelets, then separate it out and recombine it to end up with whole blood with a normal number of platelets. I was told it wasnt to do with it being dangerous or harmful for me, or the resulting blood being dangerous for the recipient, just cost and time wise it wasn't worth it for them - it would cost about 3x more than blood from someone else.
This was told to me by whoever answered the phone at the National Blood Service, I don't know how accurate it is - lot of their other rules don't seem to be based on fact. As a woman, I could have all sorts of unsafe sex with a different sexual partner daily (for free - if I was charging money that would magically make some difference to how safe the blood is, not sure how that works) and could still donate blood. But if I was a man, and had had one male sexual partner, and had safe protected with that partner, I would not be able to donate blood, I assume because people are scared of catching teh gay.