I guess because people are used to pay for phones in 24 instalments and are often upgrading after they finish paying for the current one, this way they are promoting people to continue upgrading every 24 months, 12 months would have been way too soon for most people. Offering 36 monthly installemnts would have made some people upgrading their phones less often. In the meanwhile people normally don’t upgrade computers every 24 months, it was never a thing, they are not used to it either, here they are just trying to get people who can‘t afford or don‘t want to spend $1-3,000 at once, buy their products anyway.