Because of their circuitry, LEDs are not always compatible with traditional dimming switches. ... If you'd like your LED to be dimmable, you need to do one of two things: find LED bulbs compatible with traditional dimmers, or replace your current dimming switch with a leading-edge (LED-compatible) dimmer.

For example, most LED lamps will require at least a modern triac incandescent dimmer to eliminate flicker and buzzing. Old potentiometer style dimmers will not be optimal for LED. Other LEDs. May require MLV (magnetic low voltage) or ELV (electronic low voltage) dimmers. If in doubt contact the lamp or fixture manufacturer for a list of compatible dimmers and transformers.

Incandescent dimmer will not work. It produces variable voltage which dims the incandescent lamps.

A LED lamp contains an electronic LED driver circuit which converts AC input power into DC current source and pushes a constant DC current through LED (to give you fixed brightness). When you give less input voltage, the driver tries to keep the LED current constant over a wide range (acts like a stabilizer) and our attempt of reducing input voltage to reduce brightness is foiled.

LED dimmers directly work on changing the value of current source and thus control the brightness.