There's two ways to see this:
1) No excuses. KG is a franchise player and he shouldn't miss the playoffs. If he's so good then why isn't his team in the playoffs? It's his job to carry this team wherever they go and he should be accountable for their lack of success. Bird, Magic, Jordan, Isiah, Malone etc. never missed the playoffs during their prime. He fills up stats sheets and thats it. He gets off way too easy. Kobe misses the playoffs and he's ripped for his selfishness, Garnett misses em and he's given a "get out of jail free" card.
2) The reason why Kobe is ripped and not KG is because he takes way more shots. His supporting cast is horrible. Take any team in the league and substract their single best player and the Wolves are arguably the worst. Jaric and Ricky Davis are terrible 2nd and 3rd options. They have huge holes to fill at every single position but Garnett's. In order for them to win ball games, Garnett has to literally do everything to even give em a chance to be in the game, playing the 1-spot through the 5. The Timberwolves organization has failed miserably at putting quality talent around him. The only year they did so, he brought em to the WCF.
I personally believe that it's a combination of both. although I do tend to lean more towards number 2