According to [1] IPv6 is only at about 33% penetration and this is for a backwards compatible 25 year old standard. In contrast TLS 1.3 is supported by approx. 14% of websites in 2019 [2]. So in comparison to IPv6, TLS 1.3 is being adopted quite rapidly.
TLS 1.3 was designed to be easy to deploy, and the deployment was tested before the standard was finalized.
IPv6 not so much.
Also, TLS 1.3 is an application protocol, so you only need the server and client to support it. You don't need an OS change for either server or client; although if you rely on TLS libraries shipped with the OS, you would. You also don't need network support, although if your network is particularly hostile, it could cause issues.
1. https://www.google.com/intl/en/ipv6/statistics.html
2. https://hostingtribunal.com/blog/ssl-stats/