1. I expect that some of those "deals" are fake, like everywhere in the world. So obviously a mean of payment that's not reversible is preferred by the sellers.
There are some scams, obviously, but there are also legit sellers with a good record of selling stuff without any problems. It's a matter of finding the right seller. The use of bitcoin isn't because of the irreversible nature of the network, it's mainly because it offers the privacy and anonimity that darknet markets users want.
2. Since the things sold/bought there are usually (?) illegal, linking them to somebody's name, address, face.. are clearly avoided.
And then the next question arise: how comes they don't use anonymous coins and risk (just a little, I know) to use Bitcoin?
Actually, I've read that over the last few years there's been a surge in demand of privacy coins like Monero and Zcash, but still, Bitcoin is the main currency in darknet markets. About the risk, if you know what you're doing regarding your opsec, you shouldn't have any problems.