What was different in the Middle Ages to today that contributed to the high death rates at the time of the plague?

Multiple things.

Explanation:

For one, during the Middle Ages, indoor plumbing was nonexistent. This essentially means that all waste, feces, urine, all that stuff, run through the streets. Not to mention that garbage would pile up outside too because they did not have a proper waste disposal system. Which is why it is evident just how uncleanly the people during that time were. Therefore, it makes sense that rodents would run rampant in the streets, as they were their breeding grounds. Rodents, and their fleas, as you know, are the cause of the Black Death plague. Hence why when paired with the other unhealthy and grimy conditions these people lived under, it is clear to see why there was a very high death rate at the time of the plague.