Because we now have defaults that make more sense, i.e.: 
BUILD_DCSTREAM_LIBRARY and BUILD_DESKTOPSTREAMER are ON by default.
ENABLE_TUIO_TOUCH_LISTENER and DENABLE_PDF_SUPPORT are set if the required dependencies could be found.
For reference, here is the commit:
https://github.com/BlueBrain/DisplayCluster/pull/38/files#r8822144