A quick test does bear out this claim[1], I see no Daily Mail articles with that search even though they are indexed[2]. However DuckDuckGo has similar results[3], so...
A much more likely explanation is that 1) The Daily Mail is just a shitty "newspaper" hardly even worthy of the title and not generally considered to produce high-quality content, and/or 2) that the other articles were much more widely shared, linked, etc. and much of society at large ignored the nonsense from The Daily Mail.
There's a reason The Daily Mail is generally not accepted as a reliable source on Wikipedia.
Aside: I am flabbergasted that Piers Morgan managed to survive this long at all, considering he's been one of the most disliked people on TV for a long time (and for good reason IMO).
[1]: https://www.google.co.uk/search?q=Piers+Morgan+Duchess+of+Su...
[2]: https://www.google.co.uk/search?q=Piers+Morgan+Duchess+of+Su...
[3]: https://duckduckgo.com/?t=ffab&q=Piers+Morgan+Duchess+of+Sus...