Why do we say 'commentator' instead of 'commenter'?

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.

A commentator is someone that tells you details about an ongoing event, such as a sports game, national ceremony, etc., because they can see things that you can't.

A commenter, on the other hand, is different depending on the context.

1. A commenter is someone who comments on past events , such as a news event, sports event, blog posting, etc.
2. A commenter is also a person that goes through computer programs that are poorly documented, and adds comments to describe in plain english as to what the code is doing.
3. A commenter is a critic, journalist who comments on something they've seen and they give you their interpretation of what they think would interest you.

Source.

A commentator is more than a commenter.

Traditionally, a commentator is someone who reports the affairs of the day - a pundit. A "commenter" would be a person making an observation, e.g., "Oh look, it's raining." The word "commentator" may offer a sense of being a portmanteau of "comment" with "orator" or "pontificator." It most assuredly is not(!) but for that reason it does not jar the ear.