Ranitidine hydrochloride is an anti-histamine. It has a competitive inhibitory effect at the histamine action location. Another advantage is that it does not lower serum Ca++, which you really want to keep an eye on in the case of scorpion envenomation. Anaphylaxis resulting from injected ranitidine hydrochloride is rare, and from oral it's almost unheard of.
The problem with diphenhydramine (the main constituent of Benadryl) is that it's main side-effects include anaphylactic shock, palpitations, excitation and urinary retention; all of which you want to avoid because if you're in a position where you need an anti-histamine, it's because the venom is already causing those effects.