It isn't, except when it is, and WiFi is established already. Sure, for a big industrial IoT rollout, you'd have to set up dedicated networks anyway, so you can choose them on their peculiar merits. For consumer IoT, requiring an additional hub or regional infrastructure is a losing proposition. For consumer-like commercial/industrial IoT and similar connectivity, think Redbox kiosks or fishing license machines where sites will not put the machines on their WiFi network, you might not have a good case for replacing cellular with your own infrastructure.

Where DECT might be competitive would be applications like wireless utility meters - high densities of installations where your own infrastructure could be more practical than cellular.