@StormLantern Based on popularity, Gap of Rohan and Astro Crater 4x4 are the best 4v4 maps.
The matchmaking system is introduced to create fun, interesting games between players. Typically this means the game is balanced and therefore challenging. A key component is a good representation of the skill of the player.
The skill of people in Seton's Clutch is all over the place and not represented by the matchmaking system. It is a map with specialized BOs due to years of playing. Those that happen to know such a BO can easily have 100 - 400 more 'rating' over their opponent. That makes the game (too) unbalanced, removing the challenge.
I think a 2v2 or a 1v1 on Setons is still interesting, as the meta is 4v4. But a 4v4 game on Setons with one or two setoners on your opponents team means you've essentially already lost, especially when you warp in on the beach location.