Given that the Lifetime Achievement award is usually something for which there is a notable associated guest list, I strongly suspect that this was decided, and probably that Cavendish knew of it, long before the SPotY shortlist was decided, certainly before it was announced. So it would have been inappropriate for him to be up for a vote that happens simultaneously with such a celebration of him being broadcast.
And does he really deserve an "of the year" award? One WT win, three wins in all, 325th in PCS rankings, 12th of his team and 17th of GB riders. He did something remarkable this year, but that was the end of a 16 year accumulation, not a remarkable year.