Why would a dimmer switch be rated for 600 watts for incandescent bulbs but only 150 watts for LEDs?

...and , and what would happen if the too many LEDs were in the circuit?

?2019-03-13T18:34:58Z

It was the limit from its major parts called TRIAC that is limited to operate this device no more than 600W AC power.
This ordinary type dimmer Output from TRIAC is still high voltage 120v AC which is not suitable to be used by LED device.
To dimmer LED bulb, a special made dimmable LED bulb and LED dimmer must be used together. This type dimmer output DC power to control LED device, and since the dimmer has very small space for its heatsink,therefore,150W is its limit as for now.

billrussell422019-03-12T22:59:46Z

incandescent bulbs are a resistive load.

LEDs. or rather the circuit that rectifies the AC and controls them, is a capacitive load, which causes much higher peak currents, which can exceed the ratings of the components in the dimmer unless it is derated for LEDs.

exceed the rating and the dimmer will die.