Why is a binary system used instead of decimal system when interpreting voltages?
It isn't usually done because it makes things way more complicated.  For binary, you can get away with one or two transistors to store and read a value.  For 10 values at different voltages you need several transistors and resistors (and possibly other parts as well) to tell one value from another.
Also, it is easier to transfer a simple high or low with out loss.
There are some few places where multilevel storage is used - modern nand flash can store up to three bits of data in one cell using different voltage levels - which are translated to normal binary values for communication with other devices.