I cannot find any info online or by talking with experienced players about this issue. I assume Goren set this up? Wouldn't this be more logical:
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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.
By design, no trump bids are carefully limited in range to 2-3 points. E.g., 15-17 (1NT), 20-21 (2NT), and 25-27 (3NT). That's a wide range that requires five bids between 1NT and 3NT.
So we added two sets of bids for "1.5 NT" and "2.5 NT". These are not legal bids of course, so "workarounds" were created. The way to bid "1.5 NT" is one of a minor followed by 2NT. That covers the 18-19 point range. The way to bid "2.5 NT" is to bid 2 clubs, followed by 3NT to cover 22-24. You can even argue that 1 of a suit followed by 1NT is a "0.5 NT" with 12-14 points, that is a "weak" NT.
Now we have a full range:
1NT, 15-17 points.
"1.5 NT", 18-19 points.
2NT, 20-21 points.
"2.5 NT, 22-24 points.
3NT, 25-27 points.
There are two (interrelated) reasons.
1) You and partner need about 25 points to make 3NT (a bit more if partner is "short" in points).
2) You need to allow your partner some leeway for the times it takes more than 25 points total, because he is short of points for "transportation" purposes.
When you bid 1NT with 15-17 points, partner will have about 7-8 points on average. The two of you will have 23-24 "on average," which will allow you to make 2NT. If partner has MORE than 7-8 points s/he will raise to 3NT. If partner has "less" (4-5 points), you might not make 2NT, but you should still be able to make 1NT after partner passes, unless partner has "a lot" less (0-3) points. The last is uncommon enough that most people don't worry about it.
In addition to the fact that 20-21 points is insufficient to make 3NT, you have the additional concern that you want to preserve space in many of these point ranges.
When you have, say, 18 points, that means 22 points are left for the other 3; on average your partner might have 7-8 points, then, and so you are in the "game" point range. But your partner might be just at 6, and you can only make a game with a favorable fit; or they might be at 12, and you can make a slam if you find the right fit.
Either of those requires having some back and forth, and if you start at 2NT immediately before any of that has happened, you lose the time to share information.
The 1m-(bid)-2NT pattern that shows 18-19 is valuable, thus, because it is able to show your partner your point range and distribution (just like above) while also finding out something about their hand.
You have that room to find out which bucket partner fits in, and often you don't even bid 2nt - if you fit nicely with their 1-level bid you might bid other things to show that fit; if they weak jump you can decide if you like the idea of a short-point game in either their suit or NT (depending on your honors and length in their suit); if they strong jump you know you're thinking about a slam now; if they pass you know game is probably not happening. And if they do none of those things:
Then you just show your 2NT and are happy to have described your hand effectively.
All of that would go out the window with a 2NT opening; that's why 2NT is reserved for 20-21: where it's an exceptionally strong hand that it's more important to tell Partner that "even if you have a lousy 3 count, we could still have a game, so pay attention," and you can afford to skip the lower space as you know you are going to 3NT-4M-5m most likely.