I would imagine because they can just get away with using EPO without all the complexities of blood doping. My understanding is that blood doping is pretty much undetectable other than measuring the resultant changes in certain blood test measurements. I don't think any of the major sports are tracking things like hematocrit levels the way cycling has, but I see no reason why if it's beneficial for a 110 lb cyclist that you wouldn't have basketball, hockey, football, tennis or soccer players all using them as well.
As far as I can tell, in the history of the NBA the only players who have been suspended for PEDs are Lindsey Hunter, Darius Miles, Rashard Lewis, OJ Mayo (twice), Hedo Turkoglu, Joakim Noah and Jodie Meeks. Shows how much of a joke their testing policy must be.