According to the report, it's not a snub against the convention - the studios simply don't have anything to promote. Marvel announced its slate of new films at a special event last October, while Sony and Paramount don't have any fandom-related films to show off.
Meanwhile, TVLine has posted a schedule of television related panels for the convention, and while Marvel Studios may be opting out of SDCC, Marvel TV will have a presence. Head of Marvel TV Jeph Loeb will moderate a panel in Ballroom 20, at 3 p.m. on Friday July 10, featuring the stars and producers of both Agents of S.H.I.E.L.D. and Agent Carter, so it looks like fans won't be left entirely in the cold.
Comic-Con International: San Diego will take place from July 9-12, 2015.