Why we use liquid crystals in LCDs? Does anyone know why do we use Liquid crystals in LCD's.Why can't we use transistors to switch light on/off instead of using polarizing filters to change pixels light/dark.Is there any specific reason why we use Liquid crystals in these type of displays?
 A: When LCD displays were invented back in the last millenium, there simply were no LED displays that could have competed with them. OLEDs existed in theory, but could not be built, and anorganic LEDs were way too expensive to built displays out of them. And there were problems with producing blue LEDs. The first LCD displays were also plain black-and-white displays without any backlight. They simply included a mirror at the back, so that the ambient light would either be reflected through the display a very light gray, or filtered out to display black.
Including a backlight into an LCD display was simple, straight-forward engineering. Making LEDs cheap was harder. Making OLEDs work reliably over the lifespan of a typical display was next to impossible.
As such, when LCDs with backlights got cheap enough to be able to compete with the bulky CRT displays, it was them who took the market. (O)LED displays simply were not ready at the time. LCD displays also still had quite a few problems like luminosity, switching speed, and color saturation. But they could be produced reliably in sufficient sizes, and they were the only display type that could be built into a laptop. Monitors for desktop PCs only followed later when the LCD displays got rid of the worst of their childhood problems.

So, the answer to this question is rather historic, not physical. LCDs simply were the first technology to be ready to displace the CRT displays, and market inertia favors them up to this date. (O)LED displays would be much more economical, faster switching, and provide better colors. But LCDs are established in the market, and have accumulated tons of small production cost optimizations over the years, making it really hard for any newcomer technology to compete with them.
A: Apparently it was technologically easier to have only one light source and change the the opacity of the material in front of it, rather than to have millions of individual light sources and switch them. And we actually use transistors.
This video is pretty informative: https://www.youtube.com/watch?v=jiejNAUwcQ8
A: 
Does anyone know why do we use Liquid crystals in LCD's [sic]

By definition, Liquid Crystal Displays (LCDs) would not be Liquid Crystal Displays if they did not use liquid crystals.

Why can't we use transistors

LCDs do use transistors.
LCDs compete with other types of display, such as LED displays. A chief advantage of LCDs is their much lower power requirements. This is why they dominate in applications such as digital wristwatches.
