Sprint still requires CDMA for voice service in some areas. They've been by far the slowest to adopt VoLTE of the big 4 carriers in the US, the others all fully support VoLTE in all areas.
Sprint's towers were built for it's CDMA network running in the 1900 MHz spectrum. However, their LTE network primarily run at the 2.5GHz spectrum which has far less range/requires a different cell tower spacing. As a result, they have coverage gaps on the LTE network. If they moved to VoLTE, they'd drop a lot of calls due to the gaps, so they've primarily kept voice on the CDMA network.