A P2WPKH takes up 3 bytes less than a P2PKH output (22 bytes vs 25 bytes) and is 12 bytes less than the 34 bytes needed for P2WSH (which needs the version byte, the push data, and the 32 byte hash). A lower size means a lower threshold of required fees and this helped incentive the usage of spending to P2WPKH over P2PKH (whereas if they only had P2WSH, it would be cheaper to spend to a P2PKH output).