Why is the activation energy for 1,2-addition less than that for 1,4-addition to conjugated dienes?

1,2-addition proceeds faster than 1,4 addition making excess of 1,2-adduct at low temperatures. Also, since conversion of 1,2-adduct to 1,4-adduct is faster than the reverse, 1,4-adduct is in excess at high temperatures. Both facts account for the same thing, that activation energy for 1,2-addition less than that for 1,4-addition to conjugated dienes. But why?


