Why do many people still argue about the decision to drop atomic bombs on Japan in 1945?

Your question is very general, but I will try to answer.

In one respect people still argue about whether it was the right thing to do simply because of the huge loss of life and the hideous lasting effects of a nuclear bomb. One trip to Hiroshima is all you need to appreciate this.

Another reason maybe lack of education. By this I mean Western countries and primarily the US and the UK do not teach the complete history of this event in school. It wasn't until I personally visited Hiroshima that I discovered the Japanese had offered a complete surrender conditional on the Emperor maintaining his position as head of state, BEFORE a single bomb was dropped. This is again historical fact and is not disputed by the US or UK.

The US (and nominally Churchill) refused the surrender stating that it had to be unconditional. Did it really have to be unconditional? The Emperor was the architect of some massive human rights abuses across South East Asia, but was a peaceful ending to a war a better option than the mass deaths of civilians? A lot of older Japanese citizens believe that we went through with the bombing as we had a new but untested weapon. We wanted definitive proof of how effective it would be.

Another reason (that also feeds in to Japanese belief that we wanted to drop it and test it) is that we dropped two bombs, WHY? One was quite enough and we knew it! Also we had extensive knowledge of imperial Japan at the time including up to date aerial photography. Why then did we drop nuclear bombs on civilian ares and not military areas? Was it because they were more densely populated and provided a better testing ground? We knew where the military camps and industrial areas used for the military were, but we ignored them, why?

Hope this helps.