TL;DR: For gaming, because ‘input lag’. [In a CRT] the electron-to-photon exchange happens instantly. While CRTs do have some sources of lag —namely, the time spent buffering each video frame and scanning each line of the frame from top to bottom on the screen —those delays are on the order of microseconds. When you move your mouse or press a button on the keyboard, the response time is imperceptible... By contrast,…
Input lag doesn't purely exist because of liquid movement and pixels changing colors. If it would, then an input lag of 50-100ms (which where common with older generation LCDs) - would also mean the screens response time (the time to change color) would be that high. Which would mean your picture would be a blurry mess.
The majority of input lag came from additional signal processing algorithms which were built into LCDs, which buffered the full picture for 1-2 frames. E.g. to apply overdrive algorithms, color correction, scaling, etc. Those are all running before the signal gets towards the crystals.
I think a lot of that was actually just bad design in the earlier generation of LCDs. And it was really bad, I had a screen with 100ms input lag and neither working nor gaming on it was really fun.
But it seems like manufacturers now understand the problem better and try to minimize the delay through signal processing.