Why would canonicalized URLs get organic traffic?

Question

I have a set of pages on a subdomain that have rel='canonical' links to my main domain. They have very similar content to each of the page they are canonicaled to, but I want them still accessible, just not showing up in SERPs.

In GA I'm seeing that they have about 300 organic sessions in the past 3 months.

I thought that canonicalizing them to another page made them non-indexable.

I can get them to show up in SERPs if I do a site:subdomain.example.com search, and likewise if I search for their main keyword when I do a -site:www.example.com search.

I do not think its likely that there are that many people searching in that way, however. What are your thoughts on why those pages are getting organic traffic?

Answer 1

Most of the time, using rel='canonical' should prevent duplicate content issues in SERP's. Search engines honor it, and will usually not display the page that has a canonical link pointing to a different page (which is the page that gets indexed). The fact that your pages from the subdomain are indexed means there's an issue elsewhere.

1. Check that your main pages, the ones you want indexed, also have self-referencing canonical tags. This will confirm to search engines that the main page is indeed the one to include in the index.
2. Check out your robots.txt files, both the one on your main site and the subdomain. Make sure you're not blocking your main content, and that there aren't any conflicts with what you intend.
3. Make sure that your canonical tags point to the exact URL's you want indexed, and not, say, non-secure versions.
4. The subdomain pages you don't want in the index might be referenced somewhere else on the web, including on your main site. Canonicalization usually still takes care of that, but it won't hurt to correct it. Use software like Screaming Frog or Moz's Link Explorer to see which inbound links are going to the subdomain pages, and work to get them pointing to your desired page instead.
5. Make sure that you don't have HTTP headers set that would conflict with your implementation. Since canonicals can be set via HTTP headers, use browser DevTools to fetch the headers for your subdomain pages as well as main site pages you want in the index, and look for any possible conflicts if those canonical headers exist.
6. Ask Google and Bing to explicitly re-crawl your site by submitting your XML Sitemap in Search Console and Bing Webmaster. Sometimes, a re-crawl will trigger updates and changes in the index.
7. Don't use too many similar directives on your subdomain pages. Either use canonicalization, noindex, or redirects. Make sure that none of those subdomain pages use more than one of these at a time.