All good things must come to an end, and that includes the 3G network that has been around since the late aughts. Telcos are supporting and building out vastly superior 5G networks, and they don’t want to spend time and money maintaining older technologies that aren’t used much.
Removing older 3G towers makes space for 5G equipment and simplifies network management. Plus, some 3G spectrum can be used for 4G data, although 3G can’t be used for 5G, and 4G still has some years left.