Did you use a trickle charger on the battery over the winter each year? I managed to get almost 20 years out of the Panasonic battery that was installed in my little Mazda, Miata sports car. When not being driven, I put a trickle charger on it for the 6 months it was stored each year.
Wouldn't you know the dang battery would crap out the year before I sold it! The saying goes, "if anything can go wrong...it will"!!
Guess I have to be glad it lasted as long as it did...