1. IPv6 isn’t backwards-compatible with IPv4, meaning that it’s not able to read most content that operates on an IPv4 system. At best, the user experience will be clunky and slow. At worst, instead of a webpage, all users will be able to view is a blank page.
2. No one is going to turn themselves off the Internet just by running v6, so for some years we need to run both protocols at once. We need to equip devices with both IPv4 and IPv6.
3. ISP and various providers will not be able to grow customer base because of this and will definitely change to IPv6 for growth.
4. There aren