The main reason McCain got sucked into it was that the other four were Democrats and it was embarassing to them.
from wikipedia on ‘Keating Five’-
McCain and Keating had become personal friends following their initial contacts in 1981,[11] and McCain was the closest socially to Keating of the five senators.[31] Like DeConcini, McCain considered Keating a constituent as he lived in Arizona.[25] Between 1982 and 1987, McCain had received $112,000 in political contributions from Keating and his associates.[32] In addition, McCain’s wife Cindy McCain and her father Jim Hensley had invested $359,100 in a Keating shopping center in April 1986, a year before McCain met with the regulators. McCain, his family, and their baby-sitter had made nine trips at Keating’s expense, sometimes aboard Keating’s jet. Three of the trips were made during vacations to Keating’s opulent Bahamas retreat at Cat Cay. McCain did not pay Keating (in the amount of $13,433) for some of the trips until years after they were taken, when he learned that Keating was in trouble over Lincoln.[7][33]