Thanks for the information, but what I was trying to find out is why it has to be in such a painful location? Keep in mind that I have no medical expertise, I'm just a curious layperson, so there will be much dumbing down required. 
I looked up a couple info sheets on rabies immunoglobulin at random, it seems to me that the recommendation
is somewhat different nowadays. The injections are still
required to go into
muscles, mind, so there's certainly a minimum amount of pain to be expected. Nonetheless, the top three priorities on
where to inject are a) as much as possible in and around the location of the bite, the rest b) as far away from where the
vaccine is injected as possible, and c) if you need suggestions, two nice big muscles are those your patient usually
sits on.(The immunoglobulin is only given to persons who
never have been vaccinated against rabies, or
maybe to those whose titer levels are similarly low because the vaccination happened so long ago. Basically they dock onto the virus and yell "hey, immune system, destroy
this!" in the time until the vaccine causes your body to produce its own immunoglobulin. I'ld
guess that injecting them into the blood stream wouldn't get them to where the virii are; rabies virii are believed to travel along
nerves toward the CNS. Also, intramuscular injections imply a rather slow "release" into the body; making
repeat immunoglobulin injections is rather strongly discouraged, mainly because of interference with the ongoing
vaccine injections IIUC.)