We know that the rate of a reaction $\ce{aA + bB \rightarrow cC + dD}$, the rate of the forward reaction is given by 

$Rate = k[A]^p[B]^q$ where $a\neq p$ and $b\neq q$ according to Chemical Kinetics.

However when studying Chemical Equilibrium, $\ce{aA + bB<=> cC + dD}$ when we write the forward and backward reaction rates and equate them, we write them as 

$Forward Rate = k[A]^a[B]^b$ where the exponents are equal to the stoichiometric coefficients.

How is this the case?