Writing sel4.iso on a CD/DVD vs. on an USB key

At first I had burned sel4.iso on a CD, but no computer was able to boot from it.

Then rereading the instructions, I saw it was asked to DD it to an USB key, which I did, and was getting much better results: a grub menu that disappears so fast that it only goes to loading the first module, then the boot module.

I kind of know that sectors on CD are supposed to be 2k in size, and 0.5k on most other mediums, like USB key normally... I think.

I know Linux distribution can often be put on both DVD and USB keys without thinking much.

Why (is?) Genode would be different?