Because ethereum was the most liquid platform at the time. With the crypto kittens strangling the ethereum network, plus any future congestion they could very easily snapshot the ethereum blockchain and move EOS to bitshares if ethereum network congestion threatens the ICO... now that EOS is well known and gaining significant traction as blockchain 3.0, it would be easy to drive all the ICO traffic to bitshares now that people want EOS. Before the public test net and the rise in EOS price, people were afraid EOS might be a scam and those people definitely didn't know about bitshares. The situation has drastically changed since the public test net went live and eos price has soared. It would be easy now to tell people where EOS ICO has moved and make them go get it