There is different reasons why they chose Bitcoin instead of credit cards . Bitcoin is irreversible (your can't chargeback) and untraceable (police won't be able to find them) , It's just a procedure hackers do to make sure they won't be caught.
As for buying Bitcoin then It  really depends where you live and the payments methods you are willing to use honestly. but there is Coinbase , Virwox.com , Circle.com and others. (check them out , they are legit services)