It is extraordinarily hard to catch someone blood doping, which is the reasoning behind creating biological passports. The same is true for catching growth hormone.
Crocop famously took HGH and admitted it to USADA assuming he would be caught after a random test - he actually PASSED the test and ended up accidentally telling on himself.
If you have a sophisticated plan for cheating it is very hard to catch and takes time. Suffice to say, EPO is not an 'entry level' PED and I find it hilariously unlikely Dillashaw popped for the first PED he ever took.