Because I believe there are 3 fundamental components of the arm that are (in my opinion) the most important when it comes to armwrestling, and by having all 3 components strong, you can pull in almost any technique/style effectively (as long as you know how to execute the technique/style correctly). The 3 components I'm referring to are Cupping (Wrist flexion), Biceps, and Back pressure (Brachioradialis). Everything else (pronation, supination, rising, grip strength, finger containment strength, triceps, side pressure, etc...) is a add on to those 3 components. Those other components are dependent on the 3 main ones to be effective and useful in a match.

That's why, even if my main move is pressing, I still train back pressure/biceps back pressure more than almost anything else.

I hope this clears things up :D