Sometimes they just do.
Its not often, but they can.
I have a dual coil on my bike (actually a John Deere part) and one side just shit the bed on me.
No signs of cracks or anything.....it just died as electrical parts do.
Newer ones just don't seem to last as long either, unless its just my imagination.
Look at all these stupid "coil packs" on new cars, (One coil per plug) they're dying all the time.
It could be that you have it mounted on the inside of your upper engine mount, sandwiched down between two hot cylinders/heads with hardly any air flow around it, and the plug wires are run thru your heads, blocking the air flow thru those passages there, too.....
I'm thinking that your coil might be getting a little HOT.
Try mounting it in a different location, and see if it fails again. Under the tranny, above the lower frametubes has more airflow, and it's still out of sight for the mostpart.
Coil problems include shorts or opens in the internal wiring, and cracks around the terminal. Can happen with age, getting dropped or kicked, a squirrel could even chew on it. i would consider the suggestion above about location and heat.
You can't always see a crack, but it's usually there. For what it's worth - ALWAYS carry a spare. I've always carried a known-good coil with me everywhere I go, and it's never saved me, but has saved a couple of bros (who never carry shit, but rely on me to).