Code change

The difference is due to a change in those who write viruses, said Mr Wong. Formerly the creators of malicious programs just wanted to spread fast to as many machines as possible.

"Now it's about financial gain not fun."
Seems like the fun has gone out of virus writing for some folks.

http://news.bbc.co.uk/2/hi/technology/4426078.stm

They now feel it is best to keep the virus below the radar, instead of boasting about it, which means the next generation of Trojans/Worms are going to be that much harder to stay on top of.