It’s easy to preserve the stuff that looks the same for everyone. The problem is that the web 2.0 revolution that happened in the late 2000’s would show something different depending on who was logged in, and then the social and mobile revolution of the 2010’s started hiding a lot of content behind logins entirely. So public webcrawlers stopped being able to access the main part of what people considered to be the internet, and tools like the Wayback Machine no longer capture what it was like for the typical users.