This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
It may have to do with the preference of the developers as well. But using vendor specific annotations over a config files can limit the portability of the application to a different vendor. If for example JPA annotations can be used as they are widely implemented by the vendors as it is the standard for the Java persistence.