However, most of the users of internet were not ipv6 ready until very recently. It is a simply supply and demand question, where nobody wanted to invest in ipv6

However, most of the users of internet were not ipv6 ready until very recently. It is a simply supply and demand question, where nobody wanted to invest in ipv6 infra first. Replacing networking gear for supporting ipv6 is very pricey operation too. I remember having pretty bad issues of having a dual stack in software. One example is when the python resolver library defaulted to ipv6 resolution first for a domain name, and the nxdomain to the AAAA request invalidated the name cache on the servers. When we changed this behaviour to default to ipv4 we saved an insane amount of CPU time. There are lots of issues like this one in a cloud environment, most people are never going to face this though.