he human immune system cannot effectively suppress the HIV virus on its own. What key feature of HIV makes it so hard to beat?

Answer: The viruses has a very high rate of mutation.

Explanation:

The human immune system has the ability to suppress the various types of pathogens entering the body.

Once the pathogen enters the body it gets recognized by the immune system of the body. Again when it tries to enter the body it is being killed by the immune system.

In case of HIV the rate of mutation is very high, so the body is not able to recognize its structure and so the immune system of the body is not able to kill virus causing HIV.