The publicity blurb reads: "In 1974, the world changed forever when Gary Gygax introduced the Dungeons & Dragons roleplaying game."
I know WotC has, in the past, done better by Arneson than Gygax and TSR did. But in this particular instance, Arneson is being slighted. Gygax and his "innovative ideas" are being credited for something that can either be (a) credited exclusively to Arneson's originality or (b) both of them as a collective effort (depending on how you choose to interpret the history), but never to Gygax exclusively. (Assuming, of course, that one is paying any attention to the actual facts.)
Considering they are reprinting the AD&D rules and not the D&D rules, I don't think it's an intentional slight. They did not use the term "created", they used the term "introduced", since Gygax was the one actively publishing it--he published the rules before Arneson was even involved in his short-lived involvement with TSR hobbies. And since this printing is being aimed both for Gygax are a writer and Gygax Memorial Fund, I think mentioning him would have been awkward in this particular instance.
I agree that WoTC has been better in giving credit to Arneson. However, I also think a lot of people overstate Arneson's involvement. You want to know the reason people see Gygax as more important than Arneson?
Gary Did More. Simple as that.
Gygax started GenCon, Gygax with Don Kaye actually founded TSR, he quit his Insurance day job for a risk while Dave remained employed. Gygax was the very prolific writer, contributing not just the OD&D ruleset, but AD&D, Greyhawk, numerous classic modules, and also worked hard in his executive roles.
Plus, he did more after he was gone from TSR--creating two other games, and was more involved with fan interaction in the last decade of his life. Arneson had numerous opportunities post TSR to establish his name. I do not believe his failure to do so was just because "Gygax had the publicity machine".
The primary reason Gygax gets remembered more--he wrote more and those of us who care about the writers and writing remember him.
I agree that Arneson should get credit where it is due. And I think he did get the rights he deserved in his legal battles. But I think people need to stop seeing them as perfect equals. If the fans remember Gygax more than Arneson, it's simply because Gygax did more directly for the development of the D&D game and more people loved and remembered Gygax's writing. At some point, if every time you mention Gygax without Arneson, somebody yells "don't forget Arneson", it comes across as just a little bit petty.