Oh, I think they knew what they had in Sanheim, they knew last year was an off season for him, but if they were rebuilding, did they want an 8 year commitment to a #3 D-man? They tried him in a bigger role this season, and he quickly wore down under a 25 minute workload. A good player, but one a rebuilding team probably doesn't want to commit to age 34 if they expect to be rebuilding during his prime seasons.
Same with Frost, they knew he was a solid #3, but how would he play when the team was more competitive and every game wasn't garbage time? Slow start this season raised doubts, but his play since his heart to heart with Torts shows his potential - now he has to demonstrate it's not a two week heater but him taking his game to the next level.
What makes someone like TK a safe bet (other than injury risk) is he's highly competitive, a big contract won't change that. With other young players you want to see that fire on a regular basis before making a long-term commitment. The league is full of players who got a big contract off a peak season and never played at that level again.