Jenkins interacts with an audience with a wide technological background. There are many developers who became proficient in Ant scripting before moving on to using Maven, developers who might be happier with writing an Ant task than editing a pom.xml file. There are mission-critical Ant scripts that still run in a significant proportion of organizations.
In Maven, you can run Ant tasks directly with the AntRun plugin (http://maven.apache.org/plugins/maven-antrun-plugin/), or through Groovy (http://docs.codehaus.org/display/GROOVY/Using+Ant+from+Groovy). AntRun represents a natural migration path. This is the path of least initial work.
The Groovy approach makes sense for Jenkins administrators who use Groovy as part of their tasks. Groovy, being a first-class programming language, has a wide range of control structures that are hard to replicate...