First lets clearify that
IPv6 can use subnetting just like IPv4 but with more bits being used to identify the address and subnet.
Now that that is cleared lets look at why subnetting is used less frequently with IPv6 address space.
IPv6 address space is:
340,282,366,920,938,000,000,000,000,000,000,000,000 unique IP addresses
IPv4 address space is:
4,294,967,296 unique IP addresses
The main point behind subnetting and private address space is to make more usable address space.
4.2 billion addresses may seem like alot however thats less then 1 address per person on the planet. While not every person on the planet uses and IP connection to get on the internet. A very large percentage of the population does and quite frequently connected users will use more than one ip address personally. For this reason subnetting and private addressing is common place in IPv4.
On the other hand in IPV6 could allot for much much more then 1 address per person. In fact the ammount of addresses that could be assigned by a single person would include all of the IPv4 address space. In fact that is even selling IPv6 a bit short.
en.wikipedia.org/wiki/Ipv6
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