CLECs in the US are pushing SIP based IP trunking services. Most of these trunking service providers connect VoIP CPE direct with the SIP based application server in the core of the network. That makes it convenient to offer CPE-agnostic services. Enterprises on the other hand get the freedom of using any SIP compliant device.
SIP standardization has matured over the years and enterprises deploying VoIP are showing increasing preference for SIP based VoIP solutions including the CPE. Ease of application development around SIP telephony is another reason why enterprises prefer SIP.
There are lots of devices in enterprise networks that need to be migrated over to IP and perhaps connect them direct to application servers. That requires standardized approach. And SIP is the best bet. In the actual deployments where service providers have used SIP trunking, it allows IT managers to deploy new advanced services in combination with migration from the legacy. For example it allows the simultaneous movement towards an IP network as well as the utility of new services like presence etc.