I would say the average collector is chasing sketch cards from official releases, not directly from artists who use their own cardstock. Maybe they don't want to deal with artists, or just like knowing what they are getting.
I commissioned roughly 500 cards from artists worldwide back when I was collecting, and most want payment upfront (at least partial payment). Prices ranged from $5 to $250. They generally do not offer refunds, so if you are not satisfied with what is produced, that's too bad. I was never rude about it, but there are some cards I did not like but completed the purchase nonetheless, as artistic vision has to factor in there somewhere.
Point being, on the secondary market you already know what you are purchasing.
But the bottom line is pretty much as Q stated--there is a huge amount of cards floating around on the secondary market. Companies cramming them into so many non-sports sets (and sports sets for that matter) don't help much, either.
And for anyone looking for another great resource for sketchcards, try:
http://www.scoundrelpublishing.com/spart2/
A lot of collectors and--more importantly--the artists producing the cards are there. Tell 'em weasel-king sent you.
