there was no such thing as tabard rep

In the beginning there was no such thing as tabard rep.
And dungeons never gave any rep.
That mechanic did not even exist.

In BC they introduced specific dungeon rep.
Specific dungeons gave rep for specific factions. Hellfire Citadel gave rep to Thrallmar or Honor Hold. That's it. Specific dungeon for specific faction.

In Wrath they introduced Wow Gold the mechanic of "tabard rep" but only for level 80 and Wrath heroic dungeons.

They were able to easily translate the "tabard rep" mechanic to the new City tabards and include the 60 and below dungeons.

They would have to drastically reprogram the BC dungeons and the non 80 Wrath dungeons to meld them into the tabard rep mechanic.

As the Blue said, there is a possibility in the future that this may be done.