Y Because is for IPV6 we need min of windows 7 /Vista OS we can't use it in windows xp because there are some issues are there but still so many peoples are working in windows xp itself so now just conversions are only made from ipv4 to ipv6.
IPV6 is best being implemented when there are populous IP addresses. IPV4 with 64 bit and with the 128bit IPv6 addresses, there should be 340,300,000,000,000,000,000,000,000,000,000,000,000,000 IP addresses. Most of us still think IPV6 is not desperately urgent yet. However, an anticipation is always very good first step.