This is so because the amount of energy available to the next trophic level reduces with each progressive trophic level. This is because the energy is utilised in life processes like digestion, respiration, doing work and the rest is directed towards achievinv growth and reproduction. Only an average of 10% of the consumed food is turned into its own body.
Now, taking a food chain comprising of plants, deer and tiger :
1000J --> 10% of 1000 = 100J --> 10% of 100J = 10 --> 10% of 10 = 1J.