I think two main reasons, and for sake of argument I'm assuming two equally talented folks here even if they are quite different in style.

1. Over hyping Adam. Sure it may get them on the tour or into the last half of the season but there will be push back against the hype at some point.

In addition to the push back excessive certainty about the winner by the judges can make the fans more complacent. My favorite can't lose doesn't tend to make people vote, my favorite might lose will motivate people to vote.

2. Who gets the voters as each person leaves. Sure some people stop voting but a lot of people just move on to the next favorite.

A polarizing performer (the love 'em or hate 'em) isn't likely to keep picking up votes all the way to the end. At some point they have all the people that like them. Some one that is more middle of the road or inoffensive is more likely to get the votes of those that will keep voting until the end but lost their favorite along the way. I'll vote for the one I'm kind of meh on over the one I can't stand if I have to make a choice.

I also think there may also be a story arc reason for Kris's appeal.

Small town kid who has to work and struggle to get better each week just to make it.

vs

The polished performer who may do something new each week but didn't really get better as there wasn't really anything left to work on.

Idol built the mythos that they pluck regular people out of regular life and catapult them to super-stardom. Truth is you rarely find someone that isn't doing something musical with their life that makes it to the voting rounds, and ever rarer to get them on the tour. Whether it is playing in a bar band, or running the church music program most of the time people who can perform on stage have been doing it for awhile. Especially if they can perform under high stress situation.