Because the IPv4 space has ran out. Nowadays they have to use all kind of tricks to share these addresses. With all mobile phones, and millions of new users in Africa, Asia and South America, plus the Internet of Things, you simply have to use a new system, and you have to transition to that.
In the future, people might not be able to use IPv4 at all, or all the time, and then you need your website to be available via both IPv4 and IPv6.
In the future, people might not be able to use IPv4 at all, or all the time, and then you need your website to be available via both IPv4 and IPv6.