This one is pretty simple, actually. They do obscure AdBlock from the store — if listed in order of popularity instead of being somewhat curated, it’d be at the top of the page. But to your point, imagine they did remove AdBlock from the store. Nerds would stop using Chrome. (Assuming it was not fairly straightforward to install from another source, as is increasingly true.) And we nerds tell our friends and family what browser they should be using. It’d hurt the overall popularity of the browser.
Also, it’s better for Chrome’s security reputation to allow AdBlock from within their store than to allow it from a third-party site. You don’t want to train users that it’s OK to install extensions with broad permissions from anywhere but the Chrome store!
It may become the case in the future that they re-evaluate these priorities, of course.
