Ah! Good ol' Jack Kerouac! He was one of the most popular Beatniks of the time period. On the Road was instrumental for popularizing the run-on sentence! What I mean by that is that the Beats believed in the value of a stream of consciousness when writing. I remember my professor saying over and over again when we were learning this book, "First thought; best thought." And don't we usually think in the first person tense?
Kerouac uses first person tense because he is the observer and the first-hand account witness of the goings-on that he describes. So the first person structure helps him to write whatever comes to his brain at the moment that he thinks about it; believing the whole time that raw language and thoughts could bring about a counsciousness of self not otherwise found in structure, or grammar, or rules of writing.