why isn't Luke Walton going?
Simple, Luke Walton was never the "Head Coach" of the Warriors, he was simply in charge of coaching the team during games and practices. This is re-enforced by the fact Walton did not get credit for the Warriors record during Ker's absence, Kerr got the credit.
As such, Walton was never eligible. Kerr was not eligible because he went last year.
As for the Ty Lue situation, he actually is the official "Head Coach" at the time the selection was made. Doesn't matter if he didn't actually coach a single game, he is the current head coach of the Eastern Conference team with the best record, and wasn't the All-Star coach last year.
The rules were correctly applied here, so the only argument is about the rules themselves. personally, I don't care. The All-Star game is meaningless, not that interesting, and I doubt something the coaches really want to do. Maybe Lue is excited about it, since it's his first time, but you can bet Pop is not happy about it.