Just really have to question this.
From what I recall, event chests are about 20% traitstones, 2% legendaries, 5% epics, and the remainder ultra-rares. There is a bias towards the event troop, but I don’t remember how exactly it works. You opened ten event chests, so you should have gotten 0.2 legendaries (check), 0.5 epics (also check), 2 traitstones (hey, you’re ahead of the game!), and 7.3 ultra-rare troops. Getting nothing above UR in ten tries is easier than we’d like.
I only open chests at increments of 50, and it has never failed me. Try that if you can!!