I get the graphic designer specific reasons, but I've noticed the same sort of trend in myself upon switching to a tiling wm. The truth is you just don't need multiple monitors with a tiling wm. Space gets wasted in a normal stacking window managers, and you have to manually and meticulously lay your windows out. Most people don't have the patience and just full screen everything. Having layouts optimized automatically, or with a few quick key presses makes things easier, and you wind up actually using multiple workspaces. Switching workspaces is easier than looking around at different monitors.

I do believe that people that use multiple monitors are either showing off or could benefit greatly from tiling window managers, because they'd spend less money on monitors, and less e waste and all that. Very few people have needs that actually demand multiple monitors, a fraction of the number of people that have them.