The history of Cat 6 design for Ethernet led to two different efforts of improving on earlier generation Cat 5 cable standards. Later called Cat 6 and the other Cat 5 enhanced or Cat 5E.
Cat 6 cables support Gigabit Ethernet data rates at a lower cost whilst Cat 5E lacks some improvements. It also uses a four-wire pair scheme to get high data transfer rates. The old version, the Cat 5 cable, contains four wire pairs, but it only uses two pairs.
As the Cat 5E cables were available sooner in the market and offered good performance at a more affordable price, these cables became a popular wiring choice for wired Ethernet. Thus, the price point and the slow transition of the industry to higher Gigabit Ethernet slowed the adoption of Category 6 cables.