explained, because he was going to Alaska.

True to his word, he left for Alaska, as he had said he would, in the middle of April. Krakauer writes:

By mid-April, Westerberg was both shorthanded and very busy, so he asked McCandless to postpone his departure and work a week or two longer. McCandless wouldn’t even consider it. “Once Alex [Chris] made up his mind about something, there was no changing it,” Westerberg laments.

Even when Westerberg offered to buy Chris a plane ticket if he would stay longer to work, he refused. He had made his plans and that was that.

Sadly, when Chris writes to Jan Burres and Bob that it is the last time he will contact them, probably meaning the last time before he returns from his Alaskan wilderness adventure, the words also turn out to be true--more true than he intended:

This is the last communication you shall receive from me. I now walk out to live amongst the wild. Take care, it was great knowing you. ALEXANDER.

However, even if Chris had returned from Alaska, he had followed through on his words that he was going to cut himself off from any contact with the outside world. He was a strong willed person who knew what he wanted and went after it. He didn't get cold feet or change his plans once he had made a decision.

McCandless was completely focused on traveling to Alaska, and experience the wilderness independently.  He planned on leaving in April, and when Westerberg asked him to delay his departure to work a little longer, and even offered him a plane ticket to Alaska to save some time, McCandless refused, saying, "No, I want to hitch north.  Flying would be cheating.  It would wreck the whole trip".

Also, McCandless apparently believed, in agreement with writers such as Tolstoy and Thoreau, in the value of chastity and denouncing "the demands of the flesh".  Krakauer notes that he lived by this belief as well, saying, "it seems that McCandless was drawn to women but remained largely or entirely celibate, as chaste as a monk".