Manual:$wgCascadingRestrictionLevels

Details
A page can only be protected with cascading protection if the requested restriction level is included in this array. Should be a subset of $wgRestrictionLevels. Prior to 1.22 this was hardcoded to only allow sysop-level protection.

This is intended to prevent abuse - if any protection could be cascading, users could who cannot normally protect pages could "protect" them by transcluding them on protected pages they are allowed to edit.