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Because the moon gets 14 days of sunlight in a row, which means you can't look at much every other fortnight.
That would be of absolutely no concern to whatever telescope we put up there. The reason daylight is a problem for astronomers on Earth is that our atmosphere scatters sunlight into the telescope no matter what part of the sky you look at. Thus, you can't easily see any stars in the daytime. The Moon doesn't have this problem since it doesn't have the required atmosphere.
The Sun will interfere with a Moon-based telescope about as much as it would with the Hubble. On the other hand, a Moon-based one would only be able to survey half of the sky at one time...