Just as Microsoft valiantly sought to prevent the fragmentation of web standards by bundling Internet Explorer with their incredibly popular OS, and not allowing OEMs to install alternatives?
And they could, but they're not stupid - it'd be DOA. No Dalvik = no official WhatsApp. That means next to no sales in Europe, which is the primary market for any Sailfish device. Google know that they're stopping competition by preventing the release of Android-compatible competitors.