Question: Why is it ok to preempt at the three level with a void when it is frowned upon to open a two-level preempt with a void?

Answer: The idea between the two levels is the with a six-card suit, you may very well play in a different suit (perhaps one offered by partner) in which case your void is of value.  You might open 2H with a void and partner suggest a different suit in which you have a couple of trumps and you can use your void.  When you have a seven-card suit, it is unlikely that any other suit will be trumps on the hand and therefore, the void is of little value to partner’s hand.

In other words, a void makes your two-level preempt too good because you may still be dummy!  Whereas, the value of the void with a seven-card suit is diminished because it is unlikely your hand will be dummy.

via Play Better Bridge.