single player you are given an infinite amount of time to create a replay. given an finite amount of time nothing is impressive. multiplayer you are given 20 seconds to perform the actions you want.you also need to worry about the opponents actions, how they may effect your own and how to deal with them, all within the time limit. the act of overcoming this limitation to perform the outcome you want is what makes it more skillfull.

multiplayer has more limitations to overcome to achieve the result you want, therefore it is more skillfull.