Why voltage gain decreases at high frequency in case of a transformer coupled amplifier?

Depends on the elements used in the circuit.

1.At Low frequency: The coupling capacitors are used to isolate the AC input and output from DC bias conditions for active devices. These capacitors with the input and output impedance of the active device act as a high pass RC filter, hence the gain falls.

2. At High Frequency: The frequency is high, but not as high as the microwave frequencies. There are two reasons

a>The capacitance of connecting wires are connected in parallel the i/p and o/p. When a capacitor is connected in parallel it acts as low pass filter, hence the voltage gain falls. This is when the frequency is high but not high as microwave frequencies.

b> The parasitic capacitance's of the active device are connected in parallel with the i/p and o/p terminals. They along with the device impedances act as low pass filter.