Sorry to be so late in responding to your question. I believe that there isn't just one reason, but several:
1. Everyone is in a fiscal squeeze= Bad Economy. The decline seems to be so drastic because I believe there were many Nuc Med tests being done that would not have been necessary. Now, insurance companies (and patients) are highly scrutinizing every test and opting for the cheapest route.
2. Similar to above, more and more facilities are ordering tests based on risk stratification, which tends to weed out unneccessary testing.
3. I also believe that the isotope shortage hurt our reputation for being able to deliver. Docs simply got too used to bypassing Nuc Med.
I'm sure that there are many variables that play into this, but there's my two cents.