WHY CUBASE MIXES ARE TOO NARROW ON THE STEREO FIELD?

The mix is whatever you make it – nothing to do with a DAW. Having said that, if you rely on panning [using the cubase mixer], you may want to alter your pan law settings in the project setup. A more prominent Mid drop (or more precisely, making signals that are panned away from the center of the panorama louder) can make the job of creating a wider-sounding soundstage easier. (That is, if you don’t produce electronic music where width is more commonly achieved using diff. techniques. In that case, you could just try inserting the free voxengo MSED on the master bus and bumping the Side signal a couple dBs and see if that does the job for you.)