By North Western India I think you mean Punjab/Haryana and surrounding areas? thing about Punjab is that it was located in the worst possible place in India. Right next to Punjab is central Asia which had the roughest, toughest and most ruthless warriors. That's probably why Pothoharis like Gakhars, Janjuas etc are strong people. They had to fight with these central asians while they made their way into India. Had Punjab been located in any other part of India and saw peace instead of constant invasions, then it's very likely Punjab would have been a powerhouse. Magadh and Deccan did not see the amount of invasions as North Western India did.
I don't know if you believe in Mahabharat or not, but when that war happened, all the strong armies and warriors were from North western India like Gandhara, Madra, Anga, Kurus and Panchaal wasn't too far away either.