Why are they time based? It's not that I dislike Time based runs, but it's not as satisfying as finding the minimum amount of turns for a science victory. Could they add a subcategory for Turn Based runs?

Because speedrun is by definition about time. If you want Turn based leaderboard, you can check this : http://hof.civfanatics.net/civ5/index.php

I suppose you're right. Thankyou