Because by stacking your runes and masteries with AS, u can focus on building ur AP in game and still have your AS early and late game, and if u play 3v3 or 5v5 getting boots of swiftness, phantom dancer, and lich bane, u have aquired the movement speed to go anywhere with ur passive movment speed so u can help you teammate quickly and will hitting like a truck with your blinding dart, thnx to the AP.

the main point of this build is so that u can use these items to explict hes masteries and runes, and by doing so u have the best chance as teemo in soloing, and team fights u can be a great finisher with your poision that drags on so much magic damage thanks to all of your AP.