Its a combination of factors:
1: Economy is collapsing into a depression. People who normally did not bother with stealing bikes and had the IQ to get jobs elsewhere are considering it as a means of income. These are not your usual crackheads, but people Internet aware and know the difference between a $10 lock from X-Mart which can be Bic penned, twisted or jacked open, versus an Onguard x4 or a Kryptonite NY lock.
2: Due to a shattered economy, there are more people willing to purchase bikes with no questions asked. $1500 bike for $1000? Sure thing. Few people bother with serial numbers, nor care about them.
3: Law enforcement for bikes is a low priority. Police have to deal with funding cuts, and crime rates going asymptotic. Chasing after a bike thief is not much of an interest. This is not to fault law enforcement -- its that they only have limited resources, so they have to go after worse crimes first.
4: Bikes have valuable components, so people who carry around an Allen head seat can walk off with a top dollar suspension fork, or just cut a chain off and walk off with a derailleur. Components don't have serial numbers, and if they do, they are not registered, so a guy with a pile of lightly used XTR stuff can get a good deal for it on CL, with no worry about a sting operation.
5: Bikes are becoming a fad item again, like in the early 70s. More demand means more demand for stolen bikes and components.
Victim-wise, one reason bikes are stolen is that they are under locked. In one place I went to, out of 20-30 bikes, there was one secured with a U-lock with a Sold Secure rating, 5 with U-locks at all, and the rest used cable locks. People mainly just wound the lock through the frame and called it done. All it would take would be one cut and the lock and bike is gone.