That is very strange! 
Normally, this is only available in the full version of Cubase (where the number of lines per staff is freely adjustable). And, even in Tablature mode (which correctly switches to 6 lines for guitar tablature in the full Cubase), I’ve just checked in AI, and it remains at 5 lines there.
By any chance, is the project imported from a full Cubase Project, in tablature mode?