Maybe its the casual/non fans that are purchasing greatest hits?
Definitely. I think for most artists that is the case, but it can be be better managed IMO than it is in the case of Michael. The Greatest Hits albums for Michael have gone way out of hand.
HIStory Vol. 1 already included GH disc, but that was just the beginning.
In 2003 came Number Ones. It went Nr 1 in several countries and since sold 4.7 million only in the US (despite of being released at a difficult time for Michael). It's still a good seller. Like I said it sold better this year in the US than Bad25.
One year later it was The Ultimate Collection. OK, that's not really what casual fans will buy because it's rather expensive. Though a great gift for hard core fans - very nicely done package, great selection of songs, including never before heard demos and unreleased song. So it's a bit more than a GH album and since it's expensive I don't think it particularly hurts the sales of individual studio albums - because this one will be mainly bought by hard core fans, who will buy the studio albums too, anyway.
Then half a year later The Essential Michael Jackson. Like I said it includes 7(!) songs of the 9 from Thriller. 8 songs of the 11 songs of Bad. 7 songs of the 14 of Dangerous. 4 songs from OTW. And you can buy it cheap. It sold 2 million copies only in the US. In 2008 came King of Pop. In 2009 This Is It, which sold another 2 million copies in the US.
Yes, individually most of these albums were successful, but IMO on the long term they hurt the sales of the individual studio albums very much, something that I don't like. I want Thriller to continue to sell. It does sell, but it would sell a lot better if the market would not be full of all these GHs, some of which (like Essential) include 80% of the songs of Thriller...
Also as there are so many GH albums out there, there isn't one on that the sales can be focused on. Like for example you have the Greatest Hits album of Bob Marley called "Legend". And everyone who wants Marley's GHs buys that one, so that sold like 14 million in the US and 25 million globally. But for Michael the sales of GH go in four-five different directions. Although when you count together the sales of all these GHs it's up there with "Legend" or with the Beatles '1' album. Wouldn't it look better if he had only one GH album on the market with 12-14 million certified sales in the US and 25-30 million worldwide? And that should be a bit "watered down" GH, not contain everything, so that people would still want to buy Thriller, Bad, Dangerous etc...