I wish they would give out a share to BTS/Steem holders as well, but maybe it just isn't practical in this case.
About why they don't do the ICO through BTS rather than ETH, that may simply due to a desire to make EOS more mainstream right away, rather than give the BTS community easier access. It could possibly also popularize the EOS token within the ETH community, thus gaining some inside support in ETH and neutralizing some resistance.
That would've been nice financially speaking