Question

Why transformer coupling is not
preferred at low frequencies?

Answer 1

Transformers are constructed to transform voltage or (sometimes) current. Let's look at the transformer EMF equation:

$$E=\frac{2\pi}{\sqrt{2}}\Phi f w=4.44\Phi f w,$$

where:

$\Phi$ - the magnetic flux on the core, it depends on cross-section of the core and the magnetic field $B$ inside, which is limited by approximately 1.5 teslas; thus high values of $\Phi$ are limited by properties of material, amount of material used, and cooling conditions.

$w$ - number of turns in the transformer's winding, limited by amount of material used and cooling conditions.

$f$ - frequency. Roughly saying unlimited value.

That is why to achieve the required EMF it's much more effective to increase frequency than the other parameters.