Never seen the fascination with tourneys, just another building you need to remember to swap out every 14 days. They just sit in my inventory.
No doubt good for some but it's a lot of time/effort for a time limited building.
Tourney grounds give 25% red and blue boost for QI. It's part of the house of cards where you need to be doing well in GE and GBG in order to earn the Forgotten temple and Tourney grounds, to have boosts to do well in QI. It's about time they gave Tourney grounds to all leagues, it hasn't been fair to have it only in Diamond league since the release of QI.
I would assume now that they made diamond league exclusive to guilds on 1000 LP, this is all to soften the blow to a lot of guilds used to hanging around in lower-diamond league who will be pushed down to Platinum, so that they still get the same basic reward chances they've been accustomed to in lower-diamond. The only thing exclusive to Diamond now are the 1 special tower and 1 side building that are unique to each championship.