Longevity.

I live in a country where they have shut off the 2G network. 3G may well suffer the same fate within a few years, as carriers migrate to 5G.

Verizon is shutting down their 3G network at the end of 2019

Difference in power usage isn't that great, and carriers are provisioning 4g capability - in areas where 3g is over-used, it's not being improved.