Why is there a "no man's land" between 1NT 15-17 and 2NT 20-21?
Because the level of 3NT is too high to be supported by opener's values with only 20-21 HCP. Even 2NT is challenging if partner shows up with a bust opposite 20-21. 

The appropriate way to show an 18-19 point hand using this style is to open 1 of a minor suit, and jump to 2NT over partner's response, as a forcing rebid.
Part of the challenge in bidding is that the space one might wish to use to describe one's hand also must remain low enough to allow a makeable contract to be reached and agreed. 

This is actually a newer style than the original Goren System (or Culbertson), which used 2NT opening to show a balanced 22-24 HCP. The Europeans were the first to realize the advantages of reducing this range, which then was popularized in North America by players such as Edgar Kaplan and Alfred Sheinwold (Kaplan Sheinwold system) on the East Coast, and Richard Walsh on the West Coast (Two-Over-One Game Forcing system).

Another reason for the change to 2-point ranges above 17 HCP was that 3-point ranges had been found to be too wide for accurate assessment of total partnership value. When NT is first suggested at the 1-level it is possible for one partner to inquire on minimum/maximum in partner's hand by raising to 2 NT. However that option is not available if NT is first suggested at the 2-level. Empirically, it was found that 2-point ranges were both straightforward to implement in a bidding system, and much more accurate in both game and slam bidding.