Well he might have just been older. Stray cats usually have shorter life spans not only for accidents occuring but just the strain of always living a harder life, over indoor cats. From your description of him, he seemed like a pretty battle worn cat. As cannabis suggested a war vet of sorts. Look at it this way too, by taking him in and caring for him you not only gave him love and attention he probably wanted/needed but you probably extended his life a couple years at least and they sound like they were happy years for him.

I've lost 2 cats in the last 4 years. 2 years ago I lost my 5-6 year old cat Clea (a stray I took in as well) to cancer. A year or so before that I lost my cat Bear that I had in the family all the teen years, he died of old age, he was 13 (A persian so it's normal for his breed)