I'd guess because Anvil began as a Java project, and Kotlin is what provides the sugar that allows Anko to provide a nice DSL.
Almost correct. I never thought of this kind of syntax sugar initially. But technically (and people posted an issue about it) it's possible to generate functions like linearLayout() etc from android.jar for Anvil/Java. This will work with Anvil/Kotlin as well.
I plan to keep v<Class> anyway because it works nice with custom views when there is no generated syntax sugar.