Hi,
I would say, IaC is so rappidly trending becase of "need for speed" in IT resource deployment, management and decommissioning.
When we deploye IT resources like VMs, Containers, vNets and so on, we have the option to deploye through GUI or CLI given by the Virtualization platform. For ex: we can use Azure Cloud portal or Azure CLI which are native to Azure. You can do the same with managing and decommissioning. IaC goes one step further by enabling a common languange for multiple vendors. For ex: Terraform, Ansible. They will also allow you to keep your code in a structured and secure manner to reuse and share.