*Disclaimer* I don't know if this has been mentioned as I didn't read the entire thread.**
You're seeing mass empire for several reasons. For starters the launch date of your shard means everyone who rushed Phoenix now has both EP and Thrawn, two extremely strong characters, especially when combined with a free Tarkin/Vader. This is also around the time when people start farming empire for an upcoming R2 event, and with the recent buffs to the dark side it just makes logical sense to make the jump from your original arena team.
One last reason before I go to bed! The date of your shard means there is/was an over abundance of Phoenix/rebel teams in the top of the ladder. When I finally made the leap from Phoenix I had dropped from rank 40-50 all the way down to rank 100+. Within a few days I flew right back up and sit between 18 and 25.
To summarize I totally agree that seeing the same team over and over again can get very dull, but as your shard matures you'll start to see a larger variety.