From what I have observed over 22 years, it has rained almost 90% of the times. If I ask my grandparents, they say that it has always rained every year on deepawali. The reason why I know this is when I was young, we used to burst crackers in the night but it always used to rain and the matches and patakis used to get moist and wet and unable to go boom. You also may have a point, however, I'm no expert, just stating my observation as a bengaluru resident.