Why have so many people died so often during the Hajj pilgrimage?

There are several reasons...

First, because Haj is something that should be performed at least once in a Muslim's lifetime (though it's not obligatory, it's something that is deemed very good to do), many put it off until late in their lives. Older people are also sicker people. They don't necessarily have more disease -- though they might -- but their bodies are simply more worn out.

Mecca, particularly in the summer, has a pretty brutal climate. It strains the body. High heat, the great possibility of dehydration, the fact that the rites of the pilgrimage all require a great deal of movement, massive crowding... it's just really hard on older people.

The belief that if one dies while performing Haj means a straight shot to heaven encourages those in extremely poor health to play it safe (in the spiritual sense) by playing it unsafely (in the physical sense).

Next, there's the simple fact of massive crowds all trying to perform the same rituals at more or less the same time. This is a major crowd control issue. In the past, bottlenecks in the traffic flow could and did result in crushing deaths. Thousands of people pressing against the crowd are fatal if someone should fall. Thousands of people pressing against the crowd can lead to others being crushed against walls or any barrier or obstruction.

Then there are living conditions. Those pilgrims with money or relations in Mecca can find comfortable accommodations. Others have to put up with sometimes primitive housing. Thousands are housed in un-air-conditioned tents, with awkward if not difficult access to water and toilets. At a minimum, this is additional strain on the body; at worst, it provides an environment for the spread of disease.

Finally, there's infectious disease. Today, Saudi Arabia requires health checks of all pilgrims before they leave their home countries. Those health checks may be good ones or they may be of inferior quality. The Saudis do point-of-entry screening for particular diseases, especially if there are outbreaks in different countries and may require prior vaccination or vaccinations on the spot.

But that's now. In the past, Haj provided a perfect environment for the spreading of diseases including smallpox, diphtheria, and plague. Once that gets started, in a mass of hundreds of thousands or more than a million, it'sreally hard to stop.