Because they turn down the lights!
The answer is as simple as that, nothing complicated. By the same token, the reason the world is dark is because the lights are not turned on. Or to put it in a way you may remember, folks are covering the light with gaudy lampshades.
“You are the light of the world—like a city on a hilltop that cannot be hidden. No one lights a lamp and then puts it under a basket. Instead, a lamp is placed on a stand, where it gives light to everyone in the house. (Matthew 5:14, 15 NLT)
It shouldn’t be so hard to see the connection between the darkness in the world and the refusal of Christians to come out from under the gaudy lampshades and let their light so shine that the world can see (their way) and glorify God.
My lamp shade tends to be a certain desire to be liked by everyone which makes me reluctant to take a stand that may seem unpopular, but God is helping me to work on it. What is your own lamp shade?