Chests spawn with different qualities, wood (50% of them), iron (30% of them) and gold (20% of them)
Only wooden chests can be "trapped".
Mimics have a 15% chance of spawning ONLY on wooden chests.
So, for every individual chest (assuming you open them all) there's only a 7% chance for that chest to spawn a mimic.
TL;DR - You unlucky as fuck dude : D