ICOs choose ETH because with the ERC20 standard it is both seamless and easy to issue tokens using a smart contract. However, if these ICOs fail, then it does not help ETH, in fact, it harms its reputation and hence price could also drop. Do not be alarmed though, because this happens everywhere. You go to a pizza restaurant for example and you love it, so you start going every week. After months going to the same pizza place, one of these days they give you a bad pizza,because they ran out of tomato sauce and it doesnt taste nice. It was unlucky, but you probably wont come back because you think that they do not know how to make pizzas anymore, and that is how the human brain works. Lets put it this way, we are kind of stupid living things
The same happens to ICOs, for every 1000 good ICOs, you only need 10 bad for the ecosystem to crash. This is one of the reasons why the ETH price fluctuates so much. So it is nice that nearly all ICOs decide to use the ETH Blockchain, however, it can be quite harmful if all of them do because it only requires a small mischance for the system to crumble.