Most of those still driving on the left were under British rule at some time, but not all (as correctly said already). There were many more driving on the left in the past, including European countries such as Czechoslovakia (now split in two), Iceland, Sweden, etc. In some cases (e.g. CZ) the change was made during occupation, in others in order to conform to the majority practice. Wider afield, some have changed post-colonial rule, as did Nigeria. The trend has been against left side traffic and my impression is that it is more to do with the influence of political strength rather than a question of simple practicality as to which is better. Most people are likely to be able to switch from one to the other without difficulty, although logically it seems that not needing to do so would be easiest. As a result, it would be nice if all could accept one or the other and not be different - those who have to change would resent the one-off expense, but that is about the only real consequence.