My theory is that the reason Ash doesn't nickname his Pokémon is because the creators don't want to confuse new viewers, as a result they make Ash keep the original Pokémon's names so the target audience isn't overwhelmed with all this new stuff and are put off.
This might be a wild theory but then again...It may be true :)
Most likely because GAMEFREAK doesnt want the viewers to think that Chimchar is called Joe. That is theoretically of course :D
Hope I helped!!!
PS: This Is Just an idea.
Ash is a mostly anime character, so they don't want to change the names because people will be confused.