Thank you for your question.
There are so many different types of these drugs for the same reason
that there are many types of any type of drug-- chemists have created
different combinations which qualify as different types of the drug,
have different effects and work differently in people. The drugmakers
have found that they can make a profit selling many different types of
the drug-- for example, if someone tries one kind and it doesn't work,
they may buy another kind and see if that one works for their
Specifically, for anabolic and androgenic steroids, each of these
different drugs can have varying degrees of either anabolic or
androgenic properties, which will cause each person to react
differently to each individual drug. Bone growth and muscle growth
can be affected differently by different drug combinations. Since
these types of drugs are also used for wasting diseases such as cancer
and AIDS, it's efficient to have many different combinations, as
people are often at many different stages of these diseases and need
different doses and strengths.
On the androgenic side alone, there are many different types of
androgens-- testosterone, dihydrotestosterone, etc.-- and these all
have different effects on the human body, allowing different steroids
to be marketed. People already have many different levels of the
hormone testosterone in their body, so some may only need a steroid
with a little and some may need quite a boost.
Wikipedia-- Anabolic steroid
"The mechanisms of action differ depending on the specific anabolic
steroid. Different types of anabolic steroids bind to the androgen
receptor to varying degrees depending on their chemical makeup.
Anabolic steroids such as methandrostenolone do not react strongly
with the androgen receptor, instead resorting to protein synthesis or
glycogenolysis for its mode of action, while steroids such as
oxandrolone react strongly to the androgen receptor."
anabolic androgenic steroids
If you need any additional clarification, let me know and I'll be glad
to assist you.