Mostly because they thought Objective-C wasn't going to land well with the Object Pascal / C++ communities, given those were the languages on Mac OS previously.
To note that Android Things did indeed use Java for writing drivers, and on Android since Project Treble, and the new userspace driver model since Android 8, that drivers are a mix of C++, Rust and some Java, all talking via Android IPC with the kernel.
To note that Android Things did indeed use Java for writing drivers, and on Android since Project Treble, and the new userspace driver model since Android 8, that drivers are a mix of C++, Rust and some Java, all talking via Android IPC with the kernel.