Cover-corners are a premium position in today's NFL. Those not entirely impressed by their current roster's production would be happy with one of the top-rated Corners in the 1st round. There are a hand-full of CBs that have similar draft grades, but are different enough (in build, past production, injury concerns, character issues) that GMs can focus on which one would potentially be a better fit for their defense.
The fact that so many teams have met with Kevin Johnson, who played for a relatively weak football program while only possessing very good (but not great) physical measurements/times, suggests scouts like what they see on film and what they hear about him as a teammate... a lot.
Hard to imagine the Cardinals not having him high on their Draft Board, unless they prefer to gamble with a corner prospect that has better physical measurements.