I have been noticing from a very long time, Cricket commentators read out score after every over. Why is it so?
Is there any historical significance, tragedy or story behind it?
Cricket like baseball is a sport that actually works quite well on the radio, and many commentators on the sport commentate on radio as well as TV.
In such 'voice only' circumstances you want to give your audience a summary of the score at frequent intervals, and with Cricket you have the natural cadence of the 'over' of ~6 balls, so this is often used. The habit has carried through to many TV commentators, as often people will 'watch' a long test match for example with only half an eye on the TV.
Cricket commentators read out the score so that people know what the score is. It's part and parcel of good fundamental sports commentary - reminding the viewer/listener how the game is progressing.
I am not sure if there was actually a time where the score was not given by an announcer at frequent intervals. For a fast scoring game like cricket (compared to, sy, the 0-0 draws of soccer) it's a bit of a given.