There are several reasons that can be attributed to the evolution of SDPs. In the early years of the twenty first century, the telecom vendor and service provider industry underwent a slowdown that had far-reaching impact on the way the stakeholders thought about their business. The stakeholders became increasingly conscious about saving their investments. At the same time, revenues from traditional voice-only services were diminishing. So the push towards diverse applications to enhance bottom-lines was being built-up incrementally. To address the requirement of cost effectively integrate diverse services at various levels of maturity and varied degrees of infrastructure sophistication, the telecom industry chose to take the path of building Application Servers. Application Servers were adopted first by the software industry as a middle path between extreme arrangements such as centralized intelligence with dumb terminals and distributed intelligence with resource-intensive clients.  

Servers relieve the ever-loaded communication signal processing infrastructure by handling the logic and functionality for individual applications. Application Servers based on the combination of the Java programming language, Java 2 Engineering Environment (J2EE) and the TCP/IP protocol combination provided numerous avenues to build open, standardized, scalable and flexible systems. The Application Server was the first step towards relieving the telecom infrastructure off the grip of rigid, proprietary solutions. The acceptance of Application Servers was bolstered by the rapid acceptance of mainly the Session Initiation Protocol (SIP) and Voice over IP (VoIP) technologies. 

It is interesting to note that IMS is almost entirely driven by Application Servers, which form the bedrock of SDPs as well. Thus IMS and SDP are synergistic to an extent. 

For more information, see: 

http://www.mindcommerce.com/Publications/SDP_SolutionMktOpp.php