PD don't want people spending too much time lapping by themselves in time trial trying to move up a leaderboard, they want us all in Sport Mode races so the matchmaking is more effective. It's the same reason why they only show the Top 10 in game, it's basically just to be used as a guide to what the top guys are doing and learn from them. They know if they did a proper leaderboard people would just stay in qualifying trying to move up the rankings instead of actually racing.
