I'd imagine there are a couple of considerations.
1. The cost of supporting older operating systems probably becomes prohibitive after a while.
2. Device manufacturers (and Google) are notorious for stopping updates to Android devices relatively early in the devices' life - compared with, say, the updates one would expect for years from Linux, perhaps, or Windows or Apple. Perhaps online commerce sites are understandably wary of the security implications of continuing to support unpatched systems.
Sorry I can't help more. Hopefully somebody with smart telephone expertise will be along with practical advice.
