While I do not know why the dependencies were not automatically added, I worked around the problem by adding them manually:
pitest(group: 'org.pitest', name: 'pitest-command-line', version:'1.15.0')
pitest(group: 'org.pitest', name: 'pitest-entry', version:'1.15.0')
pitest(group: 'org.pitest', name: 'pitest', version:'1.15.0')