1) interoperability between existing ecosystem and APIs

2) development environment support.

In case of Kotlin Google had to do nothing: Kotlin is 100% interoperable with existing Java code and libraries, including Android APIs; as for the development environment, JetBrains (the company behind Intellij IDEA and Kotlin itself) provides the same level of development experience for Kotlin as for Java, which means that Android Studio gets the very same nice features for Kotlin.

Now, how would they get the same with Go? For interoperability they would need to either generate tens of thousands of bindings for existing APIs and provide a convenient way to include compiled Go binary as a static library. It is highly doubtful they actually can do, since they have been having troubles to do it for C or C++ code for a very long time. Or as alternative they can improve their support for native code, which then can be used as a base for interoperation with Go code; however it is still considerably less pleasant way to do things than in case of Java + Kotlin.

Now, as for the development environment, Google will have to basically create yet another IDE on top of Android Studio to support Go; they might take an existing plugin (which is not that great, compared to Kotlin support in AS/Intellij), but it means that they would have to collaborate in development on it, which in case of Google takes too much time. Another indicator of how difficult for them implement at least someway decent support for another language on their own is situation with native code - it's being in development for like 3 years, and still is highly unusable.