Question

what explanation does smith give for the loss of men and his own capture? why does he include these explanations?

Answer 1

In "The General History of Virginia," John Smith describes the perils the settlers suffered in trying to establish the colony of Virginia.

John Smith argues that the conditions the settlers faced when arriving to the New World were extremely harsh. Within ten days, sickness settled in. The settlers did not have enough food and were waiting for supply ships, so they relied solely on corn and water. They had no shelter, and as they were unaware of how long the trip was going to be, they lost time for farming. Nearly half of them died, and it was only thanks to the Native Americans that they were able to survive.

John Smith most likely included the explanations because he felt responsible for the things that happened to the settlers, as their president. He wanted people to understand the struggles he faced. He also wanted to represent the relationship the settlers had with the Native Americans, which were not always positive.

Answer 2

Smith excuses himslef because the voyage took more than ispected and they encounter some kind of "danger" which was not in his or their plans. Apart from these inconvinients, in the new world they faced the lack of food, no shelter and its consecuences like sickness and death . 
He might include these explanation in order to portray the situation, as an excuse for being capture.