In lower metal layers the cross sectional area of the metal decreases. As the cross sectional area decreases the resistance increases. So the resistance is more in lower metal layers.
Hence to reduce the delay its suggested to move to higher metal layers.
Also lower metal layers are more prone to coupling cap as the distance between the two metal layers is less. If you want to avoid more cap move to higher layers or increase the distance between the two metals routed side by side.
From the formula below if the distance between two metals decreases the capacitance increases.
A is area of contact of two metals.
D is the distance.
E0 is the permitivity of free space in general here it is permitivity of the dielectric / the permitiivity of the material between the two metal layers.
