Excellent question, sunteam_paul.
I've wondered this myself.
Here are a bunch of factors to consider:
1. Some folks have an exceedingly narrow interest in PCE (i.e. their favorite franchise). Akumajyou Dracula X Chi no Rondo, anyone?
2. In that vein, old-skool shoot-em-up fans eventually familiarize themselves with PCE. At least these folks are interested in exploring an entire genre in the PCE library. I'm sure this applies to other genres as well... to a lesser extent.
3. Some folks have a fleeting interest in PCE. They don't stick around for long.
4. Some folks love PCE, but don't find the forums sufficiently endearing and/or engaging.
E. I thought Wii's Virtual Console would really "popularize" TG-16/PCE... and it has, to an extent, but the stagnant VC has silenced our best "promotional too".
F. The Swiss love PCE.
G. .....