EOS doesn't exist on Ethereum. It is being developed. The Ethereum based tokens just represent EOS tokens when they become available, at which point they'll be exchanged for actual EOS tokens.
I don't think EOS is dependent on Ethereum. They could probably just as easily have released the tokens on Bitshares.
This is exactly the point @southerncross is trying to make: Why didn't they?
Thank you. I am still curious as to the answer, since it would seem to also promote BTS. However, perhaps the most likely one is "it would have raised less money" because Eth use > BTS use atm.
Because exchanges are more familiar with trading eth tokens, and trade is important for distribution to be closer to PoW by controlling price by making it too expensive to buy all coins. All done with goal of security later.
Looks like both reasons make sense.