Because all the rice in his kingdom will not be sufficient to grant the wish. In fact he may not be able to find so much rice in all
the civilised world at that time. Dont believe it? look at the following table
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So you can see by the time you reach the 50th square a staggaring amount of 2.8 billion Kg of rice will be needed, proceeding further to the 64th square you can calculate the amount of rice needed. Surely it would not have been possible for the King to grant such a wish, probably even today it would not be been possible.