4chan constantly deletes its own content. There are a limited number of slots for threads, and making a new one causes the oldest one to disappear. Google does not care much for dead links.

Additionally, image submissions are basically never described in the text, so they are unsearchable even when they are live. There's some exceptions with archives but now you're in power-user territory.

Not being compatible with search engines is a side-effect that its userbase actually likes. You are either currently there or you are not.