The English word commentator comes directly from Medieval Latin commentator. However, this Latin ancestor is labelled as rare and some dictionaries don't have it.
Classical Latin does not use commentator but instead prefers commentor.
Both are formed after the verb commentārī, but one can see that by adding the standard Latin agent noun suffix ("-or") to the verb yields "commentor".
Please note that the Latin verb commentārī had a much broader meaning. It can be used as any of the followings: "to consider thoroughly [thoughts]", "to prepare [exposé]"; "to invent", "to compose", "to write [literary works]".
For instance, "commentarius" has the meaning of "memorandum , notebook". Remember for instance the original Latin title "Commentarii de Bello Gallico" of Julius Caesar's ("Commentaries on the Gallic War"): these are actually not comments but a notebook, a relation (a title designed to be neutral but with an agenda as is well known).
I'm not too sure why Medieval Latin "commentator" came to supplement Classical Latin "commentor" but I speculate that this is related to the gradual loss of meaning as "to invent" and to the consequent specialisation as "to expound" in which case "commentator" would be formed after "commentarius" the noun (this Julian "Commentarii" really looked like comments).
So we have "commentary" and "comment" (just as we have documentary and document).
Looking up both words in the Century Dictionary shows the nuance:
- A commentator "makes comments or critical or expository notes upon a book or other writing".
- A commenter "makes remarks about actions, opinions, etc.".
There's a whiff of scholarship in the commentator that is absent from the mere commenter.
I don't deny that musicality or morphological consistency have a role to play in our vocabulary. However, and this is particularly true of English, I would argue that when several words with close signification are in competition, they tend to specialise and contribute to the language's richness.
In that particular case the reason why we might be more attracted to the variant "commentator" is possibly because of its perceived higher quality standard.
Nevertheless, the word "commenter", having a long history of its own also has its dedicated niche where it is preferably used.
A significant proportion of the COCA corpus entries I found had "commenter" associated with "anonymous" or "typical": sounds better than "anonymous commentator" this time.
The simplest explanation, which does not really delve into linguistics at all, is that "ten" is not a unit that you use in multiples. That is, "20" is not spoken of as "two-tens", "50" is not spoken of as "five-tens". Thus there is no need for the disambiguation of specifying "one ten". (French seems to have decided that if you don't specify a count, "one" is assumed, as you don't say "un cent", "un mille", "un milliard", etc.)
Best Answer
Questions of why are often difficult to answer when it comes to language. Over time, people come to express things a certain way, and what begins as a personal preference or novelty may over time become convention. Anyone with small children has probably been asked why we say inside-out instead of outside-in, and responded That's just how we say it, sweetheart. Now stop changing the subject and finish your asparagus.
That said, there is a large vocabulary of terms for the multitude of shortening phenomena in languages, from clipping to contraction, ellipsis to elision; human communication is full of aphetisms and hypocorisms and abbreviations and univerbations. It seems the inclination to reduce speech to the absolute minimum needed to express an idea unambiguously is quite human and quite universal, else would we need so many words for it ;) ?
When it comes to numbers, we use explicit values where they are important, as in science and industry. But most of the time, we will understand the order of magnitude of the numbers being discussed, and can often drop them. This is stereotypically associated with Americans, and among Americans with obnoxious young workers in finance, but if a salesman tells me I can give it to you for fifteen, I understand that the car salesman means $15,000, the refrigerator salesman means $1500, and the grocery checkout clerk means $15. Since most of us live under 100 years, we'll similarly understand what century is meant. Someone wearing a Class of '21 sweatshirt will be understood to be either 2021 or 1921, with no chance of confusion for the other. The '68 Ford Mustang does not refer to a horse ridden by a Mr. Ford in 1468.
And while the Gregorian year is supposed to correspond to years since the birth of Jesus of Nazareth (as [mis]calculated in late antiquity), in practice, even among devout Christians, it is a label, not an anniversary counter. There is no theological profundity associated with the year counter (at least since the first millennium). So although legal documents and other formal communications will spell out the full year (in the Year of Our Lord One thousand Nine Hundred and Sixty-five and in the thirteenth Year of Our Reign), there is no quarter of society which demands its use elsewhere.
So if linguists tell us that humans have a tendency to shorten words and sentences, and if we acknowledge that the long form is neither necessary for understanding nor required by fixed phrasing, and we observe that nineteen ninety-three has half the syllables of one thousand nine hundred [and] ninety-three (and the dropped syllables are in the almost-superfluous century indicator), there's really no question here. Mitch gave you the answer, using a few thousand fewer keystrokes than I have: it's shorter.