The 2017 reboot runs on operating system. It is not Android, iOS, or even KaiOS (the lightweight OS found on some modern feature phones). While the 3G model has enough internet capability for basic browsing via Opera Mini, the Series 30+ OS is closed and does not support an app store or side-loading applications like WhatsApp.
Let the 3310 do what it does best: survive nuclear winters and play Snake . Leave WhatsApp for the glass-backed slabs in your pocket. whatsapp on nokia 3310
But the reason is more nuanced.