How about simply because one likes to have variety, as I explained previously.
I have an iPhone 4S, 5S, iPad 2 and iPad Air. Selling the 4S and iPad 2 would only be a gain of chump change. And I don't want to give up my most updated iPhone or iPad. I use the 5S for my second line. Was going to get the SE to replace it, but 16gb is too little and 32gb is too expensive for a phone that's not my daily driver. I had no interest in the 6, 6S, or 6S+, cause I feel I get the same experience from the 5S.
I personally find nothing major about the lastest iPhone that I like over the lastest Galaxy devices. But I do like to tinker with Android, iOS, WP, and whatever I can get my hands on.
You are looking for a definitive explaination from jamezr, which he probably doesn't have and doesn't need to have. Some users just like to have differ experiences at any given time for no reason whatsoever besides interest.