I am not quite sure what you want to do, but first of all in a [ t ] your "constant parameter (Ixx, Iyy, Izz)" are functions of t as well.
So I remove this
num = Numerator[a[t]] /. {Ixx[t] -> Ixx, Iyy[t] -> Iyy, Izz[t] -> Izz} // Expand
den = Denominator[a[t]] /. {Ixx[t] -> Ixx, Iyy[t] -> Iyy, Izz[t] -> Izz} // FullSimplify
and Collect terms
num1 = Collect[num, {Ixx, Iyy, Izz, Ixx Iyy, Ixx Izz, Iyy Izz}]
Check it
(a[t] - num1/den /. {Ixx[t] -> Ixx, Iyy[t] -> Iyy, Izz[t] -> Izz}) // Simplify
You may well replace Ixx for Ixx [ t ] ans so on in num1 / den again....
Hope this helps.