The correct answer to this open question is the following.
Claudius argued that giving citizenship and high office to conquered Gauls would be good for Rome in that giving citizenship to them represented loyalty to the Roman Empire. He did it so because Claudius thought that one reason for the ruin of Athens had been that they did not give citizenship to all the people and that differentiation affected the loyalty of the citizens to Athens. Claudius did not want that to happen him and that is why he offered citizenship.