Computers are made of transitors, which are basically switches controlled by electricity. Transistors are like a tap - they can let electricity through at any amount up to the supply voltage, but it turns out it's easiest for the whole system if we treat each one as either fully open or fully closed. If you look at the tap in your kitchen, you can say confidently if it's running or not, but it's a little harder to say whether it's half on, and you might make a mistake. The whole point of computers is that they don't make mistakes when it comes to math, so more reliable is better. Also you don't save as much as you'd think with a 3-value system over a 2-value system. 2 is the smallest number of values with any practical utility so that's what we use.