system
Given the following post, respond as if you're commenting on Reddit:
human
{Star Trek}What happened to the rank of Commodore?
gpt
The rank of Commodore, in Starfleet, is given to an officer who runs a task force of ships.  Unlike Admiral, the Commodore is the captain of his own ship, but he is the flag officer for the rest of the ships in his task force.

In the 23rd century, task force deployments were common.  The Constitution class was relatively unique in that she could travel solo, something the Federation liked for deep exploration ships, but much less so for everything else.  Federation task forces usually had a number of ships in a group, usually grouping one kind of cruiser with several smaller operations ships and some escorts.

Starfleet's 24th century paradigm is different.  The success of Enterprise (NCC-1701, not NX-01) at long, sustained exploration, combining heavy weaponry and yet a full suite of science stations, meant the idea of a single ship rose again.  The general success of replicator technology meant that Starships could more easily go on lone deployment, even in deep space.

With the decline of the task force, the rank of Commodore ended up less and less meaningful.  With ship groups formed dynamically and on an ad-hoc basis, captains would be given short term flag status when need be, but none of this necessitated a new rank.

Admiral James T Kirk was one of the first to be promoted directly from Captain to Admiral.  Making him a Commodore would make no sense -- NCC-1701 needed no escort nor was it designed to be part of a task force.  Moving him to an older ship would be a demotion.  Thus he was promoted directly to Admiral once it was determined he was ready for a fleet command role.  (This was a terrible mistake, of course, but that's another story.)  Anyway, before long, this was standard convention.  Did you captain a starship?  Straight to admiral.

The rank of commodore officially exists, but until Starfleet starts making ships to work in task forces again, it's unlikely we'll see it in use.