As far as I can make out, the new website was considered a key part of the "Rust 2018" project, and so the maintainers chose to launch it before it was really finished (and without the planned period for user feedback), at the same time as the Rust 2018 release.

Evidently it wasn't quite such a key part of the project that they could consider delaying the new Rust edition until the website was ready.

In hindsight tying the website update to the new edition was quite possibly a mistake. The Rust core team has asked people to hold off on discussing the process or proposing major changes until they've published a retrospective on what happened (that was five months ago, and the retrospective is now "mostly finished").