One reason being, he is too young. The experience of John Paul II should be sobering: only 3 of the cardinals in the Conclave that elected him where still eligible to vote the next time around. And one of these three (Jaime Cardinal Sin of Manila) was so infirm he never made it to Rome, which left just two to actually participate. Granted, one of the two got to be elected Pope (the other was the American, William Cardinal Baum). But that was luck. For most of them it should be clear: you elect the young guy now, you won't be the Pope yourself.
Which is sobering a thought for them but not perhaps great practice in the long term. Stability, which is what the Church really needs right now, is not best given by way of four and twenty 80-year-olds being bumped off every five years in the name of ambition (or something like ambition).
