Because people actually want to buy fentanyl laced products. I've known people addicted to stimulants that they buy knowing that they are cut with fentanyl just to get that combination with the opioid high.
Do you think that's the case for cocaine? Personally, I have a hard time believing that because of both the type of high cocaine is for and because that seems like a worst case for a mix since mixing fentanyl in a powder seems like it would be error prone to me, but I can't deny that people do crazy shit.
Yes this and also some states are highly over taxed / priced so people are willing to pay competitive street prices in blind faith.