Chris wasn't aware that there was another way he could have crossed the river back to civilization. Describe how he could have done that in Into the Wild.
A passage detailing Jon Krakauer's own journey to the bus where McCandless died explains that there certainly was a way in which the young man should have been able to escape his death sentence.
Unlike McCandless, however, I have in my backpack a 1:63,360-scale topographic map (that is, a map on which one inch represents one mile). Exquisitely detailed, it indicates that half a mile downstream, in the throat of the canyon, is a gauging station that was built by the U.S. Geological Survey...
We arrive to find an inch-thick steel cable spanning the gorge, stretched between a fifteen-foot tower on our side of the river and an outcrop on the far shore, four hundred feet away...hydrologists traveled back and forth above the river by means of an aluminum basket that is suspended from the cable with pulleys...The station was decommissioned nine years ago for lack of funds, at which time the basket was supposed to be chained and locked to the tower on our side--the highway side--of the river. When we climbed to the top of the tower, however, the basket wasn't there. Looking across the rushing water, I could see it over on the distant shore--the bus side--of the canyon.
This passage is found near the beginning of Chapter 17 and does provide additional information about the basket and pulley.
I think we also need to question, however, whether or not Chris would have had the strength to cross the river in the cable cart near the end of his life. As malnourished and weakened as he would have been, it would have been very difficult to accomplish the feat in a hand cart.
The simple truth is that we do not know for sure whether Chris was aware of the cable cart or not. Being only half a mile downriver, it would stand to reason that he was at least cognizant of its existence; he may, however, have known he did not have the strength in his final days to accomplish the crossing.