J2EE has emerged as the standard for Enterprise Applications for the following reasons – 

The Enterprise Application Challenge
J2EE standards provide user interfacing, data enabling, communications enabling, system assurance services, web enabling and application enabling, which constitute the critical requirements for any Enterprise Application architecture technology. Enterprise System development has benefited with the use of components model, one of the core underpinnings of Java Enterprise development and the J2EE. Features such as transaction processing, security, object persistent, resource pooling etc have helped in scalability and performance for Enterprise Applications.

Speed in Application Development
The J2EE Enterprise Java Beans (EJB) Component Model serves as the core way in which distributed Java Enterprise Applications can be rapidly built. The Component based model enables Enterprise Software to be developed more rapidly with high reliability. Components have a coarse-grained nature that facilitates more rapid application development. EJB clients talk to EJBs using standard interface patterns and also operate inside of their own J2EE application client container environment, which helps in the development and implementation of server components a simple task. J2EE servers allow fast development of complex applications capable of withstanding many millions of transactions per second and many hundreds of millions of users.

Simplifying Application Deployment
Leading Application servers today such as WebSphere, SunOne, Weblogic etc. support J2EE and are built with features comparable to UDS in deployment of complex applications. They provide functionality such as load balancing and failover through clustered services, messaging mechanics using Java Messaging Service (JMS), concurrency achieved through Entity beans, easy-to-deploy feature by its Deployment Wizard etc, which ensure successful deployment of enterprise level applications.

Enterprise Application Integration
Prior to the advent of Web Services, Enterprise Application Integration was very difficult due to differences in programming languages and middleware used within organizations. The interoperability was cumbersome and painful. With Web Services, any application can be integrated so long as it is Internet-enabled. The foundation of J2EE is XML messaging over standard web protocols such as HTTP. The service's operations are generally invoked using SOAP (Simple Object Access Protocol) and ebXML, which is emerging as a very important standard for eBusiness collaboration. ebXML is a suite of XML specifications and related processes, and is behavior designed to provide an e-infrastructure for B2B collaboration and integration. SOAP offers an XML format for representing parameters and return values over HTTP. This is a very lightweight communication mechanism which helps integrate businesses, departments, or applications quickly and cost-effectively by easing interoperability greatly.