I'd question the "always" part, really. Kirk's ship in TOS never seemed to combine the jobs, and even when the TMP bridge set seemingly gave both jobs to Chekov's console, it still remained possible for Sulu to fire the guns from the helm console in TOS fashion.
The two job titles were explicitly combined in the persons of Worf and Tuvok. But Tuvok was a casualty replacement - he need not have originally been the Chief of Tactical, only the Chief of Security like Janeway refers to him before the Caretaker incident. And Worf, too, filled in for the fallen Tasha Yar in the Security Chief role. Since the division of labor in the first season of TNG was a very confused issue, it could well be argued that Tasha Yar never was Chief of Tactical; we could argue that Worf was, and then took a double job at Picard's special dispensation.
As to why things were so messed up during the first half of TNG S1, remember that the ship was seriously undercrewed when sailing to Farpoint. Perhaps she remained so for the first half of the season, due to Starfleet cancelling the great exploration mission beyond Farpoint once it became evident that there would not be a Farpoint to support that mission. The crew intended for the E-D might have been assigned elsewhere, never being shipped to the waypoint that had suddenly become a space mollusk and floated away. This would explain e.g. why the character of Colm Meaney had to wander the corridors in Security tasks, despite later being revealed as transporter engineer O'Brien...
Essentially, every other ship in Starfleet might have had separate Tactical and Security department heads. There's enough evidence that prominent skippers "customize" their bridge crews and even their bridge architecture; the combined Tactical/Security might be a pure Picard thing, plus something forced upon Janeway due to manpower shortages.
Timo Saloniemi
