The reason for KK not being rerouted was that none of these places were major places back in the day. They were just relatively large towns. Since the late 90's, they have developed rapidly, but one must remember that back in the day when KK and the other trains were introduced, Bangalore and Mysore were pretty much the only major places in the entire state with out-of-state passenger traffic. Even if these had been converted to BG at that time, no diversion would have been done.
Even KK's diversion via Daund got many objections from Karnataka politicians themselves as they felt Bangalore would lose a premium SF connection to Delhi, while the other three Southern states' capitals would continue to have their...
more... respective SF trains to Delhi (AP, Kerala and TN). For this reason, between 1982 and 1989, there were 3 SBC-Delhi trains running through three routes - via BZA, via SC (Rajdhani route) and via Daund. It was fully diverted via Daund only after some reassurance that a new train would be introduced in the faster route via SC.