It is all over the forum, but it is also in the FAQ,
https://vborg.vbsupport.ru/showthread.php?t=234458
Q8.
Why does vBulletin.org not use the latest version of vB?
A8. Lynne made this post in a locked thread stating why. Asking this question will just result in a thread lock and the following reason being supplied:
The question about whether vb.org will be upgraded or why we have not upgraded has been asked and answered over and over again. Please do a search. The answer is actually very simple - the site works just fine as is, so we don't feel the need to upgrade it.