The reason being that you don't know exactly how your body is going to respond to synthetic testosterone.
Perhaps you are one of the unlucky few who get gynecomastia from even mild dosages of testosterone...If that's the case, you will know exactly what drugs you should take to stop the gynecomastia without guessing.
When you add in multiple compounds without knowing what your body accepts / rejects, you won't know which is causing your problems, thus making it more difficult to solve.
Deca is a 19-nor compound that has the potential to increase progesterone and prolactin in the body...Testosterone carries the risk of increased estrogen levels...Both compounds require different treatments to reverse gynecomastia symptoms.
When you finish your first cycle of only testosterone you'll have the knowledge to save your time, money, and worry about any complications that arise.
Best of luck, and please consider running just the testosterone @ 500mg/EW. That's all you'll need to reap the benefits of steroids for your first go-around.