1. Anabolic steroids are either completely banned or on some kind of controlled substances lists in all developed countries. Leaving traces of transactions for steroids in your bank account or credit cards statement is not a good idea. If you do it just once is a lifetime OK, but if you buy occasionally or even regularly authorities may suspect further resale beyond personal use tolerance legislation and act accordingly. Bitcoin is totally anonymous and they can not prove what did you spend your bitcoin at specially since receivers wallet is also anonymous and not associated with steroids sales officially.
2. Bitcoin takes just minutes for transaction. Plus you can do it online, do not need to go to bank and spend time there. By far fastest and most convenient payment method.
3. Credit cards are simply banned to use for steroids purchases, it has been so for longer then a decade now. Merchants try to overcome this situation in many ways, mostly with presenting something else as purchase items, not steroids. This works to a degree but is associated with much higher costs, so anyone buying steroids with credit card is paying much more. You can always get same product as lower price if pay for it with some other payment method, specially bitcoins. With bitcoins merchants have less expenses and so can offer steroids at lower prices. Simple as that.