Dirac had proposed that Gravitational constant would reduce with time.Why?
If you mean, why did he propose that, I can't say. If you mean, why would the gravitational constant reduce, it might help if you gave a reference to what Dirac actually proposed.
It wasn't a serious proposal. There are some really huge numbers in physics. One of them is the strength of the electromagnetic force between electron and proton compared with the strength of the gravitational force. Another is the age of the universe in Planck units. Dirac just speculated that these numbers were related by some undiscovered law. If they were, then that would imply that as the universe gets older, the strength of gravity weakens (compared to electromagnetic forces).
This is described on this Wikipedia page:
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