I heard israel is very much advanced in medical sciences...why it is not the top medical tourist place? Is it because it is expensive as compared to asian countries?

It is because of the war with Palestine. The war has affected Israel tourism including medical tourism. You are right- Israel is evry much advanced in medical field. But due to war, most tourist prefer to go to stable country where they can combine medical tourism with vacation.

I don't think it is only this.

Prices in Israel is much more expensive than in some countries of South America or Asia

The war and terroism are not the primary reasons, large numbers of people visit Israel for tourism. Israel has traditionally been a place for tourism based on religion. Places holy to Judaism, Christianity and Islam are the in Israel, and people visit the country for this reason. Medically though excellent, it is not cheaper than countries like India, Thailand and Philippines, which also provide excellent medical services and are excellent touristic destinations too.