This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori 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.