Why does Giovanni, a leader of criminal group, a wanted person, decide to join a stupid gym leader tournament in the first place anyway?
You could ask similar questions about a lot of leaders. Why would Volkner travel to unova when he could(and should) be maintaining Sunyshore City through Sunyshore tower?
Not to mention how haven't they aged like Caitlin has over the past 5-9 years or so. If Blue was 13 in HGSS, how is he's exactly the same here.
Honestly, I think no reason will be provided as to why the Gym Leaders are in Unova or why none of them have changed in the past few years at all.