I stand (sit actually) corrected. I only saw the terminator entries from WD and like I said the first specific GK models were the metal terminators. However, my point stands. The army was very much fleshed out even back then and it was less than two years after Slaves to Darkness when the Terminators were introduced (1988-1990). They were never the "bits & bobs" that the previous poster claimed. They have always been their own chapter in the same manner as the Sororitas have been part of the Ministorum. And while Inquisitors could be bought with the GK, again very soon after Slaves to Darkness was published they were made available for other armies as well. If anything it was the Inquisition that was the least fleshed out of any of the elements currently occupying space in the GK codex. That's why to me it makes perfect sense to split them off into their own armies.
Completely off-topic: making me go back and read those old books makes me miss a lot of the old flavor the game used to have. Genestealer cults, Exodites, specific Traitor Legion lists, Imperial Robots, etc. Granted, a lot of the old models were hideous compared to the amazing stuff GW puts out nowadays (barring my negative opinion of the new chaos dinobots ), but the backstory contained in a lot of ways so much more in the way of ideas and creativity. JMO though.