Why doesn't my best score value load after game closes and launches again? (Swift)

My best score value works perfectly fine if the game keeps running, but it won't load the best score if the game closed and was to launch again. It goes back to zero every time it the launches again.

Here is what I'm doing:

import SpriteKit

class EM: SKScene, SKPhysicsContactDelegate {

var bestScoreText = SKSpriteNode()

let bestScoreCategory: UInt32 = 1 << 4

var bestScoreLabelNode = SKLabelNode()
var bestScore = NSInteger()

override func didMoveToView(view: SKView) {
    /* Setup your scene here */

    var bestScoreDefault = NSUserDefaults.standardUserDefaults()

    if (bestScoreDefault.valueForKey("bestScore") != nil) {
        bestScore = bestScoreDefault.valueForKey("bestScore") as! NSInteger!

    }
.......
....... 
     bestScore = 0
  }



func didBeginContact(contact: SKPhysicsContact) {


    if (score > bestScore) {

        bestScore = score

        var bestScoreDefault = NSUserDefaults.standardUserDefaults()
        bestScoreDefault.setValue(bestScore, forKey: "bestScore")
        bestScoreDefault.synchronize()

    } else {
......
}

via Chebli Mohamed