Why no species fanbois in Star Trek?

[Therin of Andor]

A human inmate in "Whom Gods Destroy" wears an Andorian suede tabard. I often wondered if he originally owned the fluffly pink bathrobe worn by the facility's Andorian inmate.

A group of dark-skinned humanoids in ST IV wear Zaranite clothing (from TMP). Meanwhile a humanoid woman in ST IV wears Arcturian clothing (also from TMP). Actually, that reminds me, in "The Conscience of the King" - at least in the script - the play of "Hamlet" is being performed in Arcturian-styled clothing.

An Andorian Fast Food outlet (Shop 01-754) is located on the DS9 Promenade [as seen on the directory board in the television special, The Science of Star Trek].

 

[The Wormhole]

There was also the human defector in TNG who took off to Romulus and later followed Spock and his movement on Romulus.

Who? I don't recall a human defector who showed up with Spock in Unification Part II.

Ensign Stefan DeSeve, who was actually in Face of the Enemy, though he told Picard he was with Spock's movement.