Why 'popular' is not preserved on query string when I change pages

This question is from the 'Hands On: Community Contributions' series, episode 12: 'Sort By Popularity'. When I am at the route: links-sharing.test/community and I typed '?popular' at the end it shows me a list ordered by most popular at links-sharing.test/community?popular. But When I click on a page on the custom pagination it doesn't preserve the '?popular' in the query string when I move from one page to another. Any idea for this behavior?

On my view:

...
{{ $links->appends(request()->query())->links() }}

On my controller:

$links = (new CommunityLinksQuery)->get(request()->exists('popular'), $category);
...

On my query class:

    class CommunityLinksQuery 
    {
        public function get($sortByPopular, $category)
        {
            $orderBy = $sortByPopular ? 'vote_count' : 'updated_at';

            return CommunityLink::with('votes', 'user', 'category')
                ->forCategory($category)
                ->where('approved', 1)
                ->leftJoin('community_links_votes', 'community_links_votes.community_link_id', '=', 'community_links.id')
                ->selectRaw(
                    'community_links.*, count(community_links_votes.id) as vote_count'
                )
                ->groupBy('community_links.id')
                ->orderBy($orderBy, 'desc')
                ->paginate(3);
        }
    }