I think the sheer nostalgia of a player actually touching their own card is the driving force. Also, the rarity of seeing a player and having the card with you in hand to have signed increases the rarity of many signed cards. The masses are realizing there are not many around.
"We" have built "america's toughest signed card want list"
from many autographed card collectors who have been at this since the 50's. Most all are missing the same cards, and that tends to create a bidding war of all wars. However, I have been contacted by a few people, not on this site, who are interested in starting a signed set. There are also player and team set collectors venturing into the world of signed cards.
I think signed cards have still not reached their potential, and there are steals to be had on ebay and auction houses of signed rarities going unnoticed. The HOF rookie cards should lead the way with others being dragged slowly up with them.