The reason is that those devices are not upgradable to iOS 11.
In an update published on the official Pokemon GO website, Niantic confirmed that it would soon be ending support for Apple devices that are "not capable of upgrading to iOS 11".
Wondering if your device is included in the purge?
Once the update is released, Pokemon GO players on outdated devices will no longer be able to access their accounts or use PokeCoins.
In other Pokemon GO news, more and more players are getting sick and exhausted of the EX Raid system.
Pokemon GO recently released a rather important update to the games Master File which could hint at some big upcoming changes to the game in the week ahead.
The move is reportedly being made so that new unannounced features of the game can be implemented without the fear of problems and bugs from interactions with old software.
Pokemon Go will discontinue support for these devices on February 28th, 2018. This comes almost a week after the third beta was released to the same party.
In addition to the above, it's also worth noting that the game just relased another batch of Ex-Raid passes. Way to kill my motivation on this game.
This week the folks at Niantic announced an update to Pokemon GO, an update that might not sit well for users of elderly iPhones.
The improvements involve a new AR Plus mode which takes advantage of the more advanced AR capabilities that were introduced with iOS 11 and ARKit.