One major reason to have a separate proposal is that there are lots of Ethereum-specific questions (i.e. about smart contract design and general Dapp design) that will probably be considered "programming technique" and therefore off-topic for Bitcoin.se, but that would certainly be on-topic for this proposal. The top five example questions, for instance, would probably all fall in this category, and I count about 10 more among the other example questions. Rather than have those Ethereum questions on Stack Overflow or Programming.se, and non-programming related Ethereum questions on Bitcoin.se, putting them all in a single place will enable the site to attract the right experts who can answer both types of questions effectively.
The Ethereum proposal also has a different audience than Bitcoin.se, as we can see from the fact that only 5.8% of this proposal's followers have been active there, and that roughly half of the proposal's followers have not previously created Stack Exchange accounts of any kind. Probably, this is because Ethereum is not "about" cryptocurrency in the way that Bitcoin and Bitcoin.se specifically aim to be. Ethereum is about creating a platform for further distributed development--the cryptocurrency ether is really only a cog in that larger machine, rather than the focus. This means that we can expect a different nature of question to dominate the new proposal, and this characteristic will only grow as more and more Dapps are released on Ethereum, making it more "app-store-like" and less "currency-like". Ethereum is designed to be a very large ecosystem, and the universe of Ethereum-related questions will diverge further and further from current Bitcoin.se content over time. Does Bitcoin.se want to try and encompass all that?
One final, if small, consideration: because of the name "Bitcoin.se", it's confusing for inexperienced users to think of that site first (or to google for it) when they want to ask a question about Ethereum. If the site was "Blockchain.se" or something like that, then this confusion would be reduced or eliminated. But as someone who spends a lot of time explaining concepts to people, it's highly confusing when I first clarify that Ethereum is a totally different network than Bitcoin, with a different objective and a different blockchain; then I tell them to go to Bitcoin.se to learn more about it. Is Bitcoin.se willing to undergo a name change?