They have never exculded the eliete four in any region. The reason that Ash hasn't faced them is because in order to challenge in the 'Champions League" a trainer must first win their region's tourtament. For example if I originated from Johto, I could only challenge the Elite Four when I acumulate all 8 Johto badges and win my region's tourtament/conference.
This was breifly explained by Lucian in one of the Episodes. I think it was: "An Elite meet and greet."