Why transformer not allow DC supply?

Transformers require changing magnetic flux to operate. DC current provides a steady current, with no changes in magnetic flux that can be picked up by the output coil.

Alternating current has continuously changing direction which produces continuous changes in the magnetic flux. This induces changing voltage in the output coil. This is (or was) the huge advantage of AC power systems worldwide: The voltage could be easily changed from high voltage for generation and transmission into low voltage at the consumer. DC generators had to run at the voltage required by the consumer and powerlines could not be higher voltage / lower current than the consumer.

Today, switching systems easily convert DC to other DC voltages, DC to AC, etc. This is done in most computers, telephones, stereos, etc. It can even be done on a grand scale, allowing use of high voltage DC power lines in selected places.