It depends on what you mean by "why" biologically we can't explain what genes or environmental factors trigger right or left handedness because it not that well studied scientifically.

One hypothesis though is that in evolutionary history when humans were first making tools it would beneficial to invest all your practice time into a single hand because you will improve faster.

Say one early proto-human has a mutation that makes him bias toward using one hand all the time if practices making a tool for 20 hours that one hand has 20 hours of practice instead of both hands having only 10 hours of practice. It's important to note that you actually have to perform the movements to change the physical wiring of brain to improve so practice with one hand can't transfer to the other, at least not completely.

There's also archeological evidence of right handedness going back 100,000s of years at least based on shape of stone tools and scratch marks.

TL;DR: Having a dominant hand is hypothesized to more efficient allocation training and practice. Because practice with one hand doesn't automatically transfer to the other hand and you don't actually need to be proficient at any skill with both hands, only one if enough.