It is already offered in the form of Ranked season which are odd-Tier, Clan Battle seasons, and even Brawls that have those conditions. There are people that enjoy the content in specific, but none of those modes overtakes the popularity of Random Battles. Splitting queues divides your playerbase further which causes wait time and access issues.
Brawls and other events are Time-Gated in order to help the modes have enough players to function. Time-Gates help normal queues populate outside of higher population windows of time, but also increase engagement for the off-mode during the window of availability since people try to take advantage of what's available while it's available.