What is the key reason ipv6 is not yet used widely?

1. IPv6 is not widely used till now because all of our networks which already exist are built upon IPv4 and we don't want to bare cost of upgrading it that's why we are still  using ipv4 networks .

2. In our LAN we still prefer IPv4 because it is easy to understand and we already have hands on practice to implement it.

3. But IPv4 last subnet is already assigned so now onwards New Public Networks will be built upon IPV6