Because you can build all the experimental smart contract ICO tokenized apps on ETH. So many of them in the pipeline now. Ethereum is possibly winning the race for developer mindshare.

Right now we claim that none of these apps will ever get any real world adoption, but we forget the speculators are the users. And now the developers are the users. Programmers matter A LOT.

I don't think Ethereum's technical design and ICO model is the best possible one, but there is nothing shipping now (or about to ship this month) which is unarguably better in app developer mindshare.

Who is in second place? Bitcoin or Steem?

And one or more of those ETH apps may end up causing a mania again. May not happen, but the chance of happening is one of the reasons to hold ETH.