Because of the differences in how incandescent bulbs work vs LED and Florescent, and thus how their dimmers have to work.

Incandescent: you just heat up a bit of metal until it's glowing hot. Literally the same effect as leaving something metal in a campfire or furnace until it glows, just super hot so super bright and white.

Florescent: have a tube of gas, and then put a super high frequency voltage across it (thousands of Hz), and enough energy will be imparted to the gas molecules that they will emit photons.

LEDs: apply a constant DC current to a bit of a custom grown semi conductor, and that will give the semi conductor atoms enough energy that they will release photons.

The main thing about those, is that for Florescent and LEDs, they require very specific types of power.

• Florescent bulbs require a very high frequency voltage, this is what the electronics in the bottom of a CFL do, convert the 60Hz of AC power from your house into super high frequency voltage.

• LEDs on the other hand, can't use AC, and need constant current applied, not constant voltage, so all the electronics in the bottom of those bulbs work on converting your house's 60Hz AC power into like ~24V of DC power.

• Incandescents on the other hand, do not care what type of power you put into them since any type of power can make them hot as long as you put enough in. You can feed them high or low frequency AC, you can feed them DC, you can feed them constant current, or constant voltage, they do not care, as long as you put electrical power into them.

Now it comes to dimmers. If you want to reduce the brightness of an incandescent bulb, you just need to reduce the power going to it, which in a constant voltage AC system like a house, means reducing the voltage.

The first dimmers did this by putting a variable resistor in series with the bulb. When it's resistance is zero the bulb is at full brightness, when it's the same as the bulb it's at half, and when it's resistance is way higher than the bulb's then the bulb is super dim.

This is good because it's super cheap and easy and you can precisely lower the voltage while maintaining the exact same waveform, but the problem with this is that you're feeding the same amount of power to the circuit no matter what, the resistor is just burning up the excess and turning it into heat.

So then we landed on how we built the vast majority of classical dimmers you see today: switched dimmers. Since incandescents are just hot metal, and there's a lag between when you heat metal and when it cools, you don't actually need to give it a clean wave form. Modern dimmers just switch on and off really fast to reduce the average amount of power going to the bulb. At 50% brightness, the dimmer might be switching on for 20ms then off for 20ms then on for 20ms then off for 20ms.

It's a great simple solution for incandescents, but when you try and feed the precise electronics of an LED or Florescent, that messy, choppy signal, they can't handle it and often just see it as the power coming on and off or it messes with their internal circuitry.