Spoiler:
Because, there is a chance they will be worshipped like the Ori when interfering, and they already saw what happens when that happens (i.e. the Ori and how they got corrupted even though they started with the best of intentions). And the Ancients believe in not interfering partially out of belief in free will, but also because they were more in line with science so perhaps they observe what happens (like reading the paper in the ascended coffee shop where Daniel was).
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