The guest list included key representatives of the major vampire factions AND key representatives of the many of the major supernatural factions.
Mara and Ferrovax were probably too senior, but then again their factions have very small numbers. So it is not that unusual. Everyone else sent a representative which is probably normal business. Given so many powerful factions were here, each representative had to be fairly senior. From an "optics" standpoint, the attendee list mostly made sense.
Of course, the political intrigue behind the scenes determined much. Mara was already training Bianca and actively plotting with her. Dresden was there as it was a trap. Thomas as an assassination attempt by his father and Lea for many reasons (Harry, Thomas, Politics, etc).
The real question is why no rep from summer? Or Jade Vamps? Or a warden from the White Council? Those all seem suspicious oversights although Jade vamps are probably explainable due to simple isolationist tendencies. The explanation is probably some variation of deliberate choice and/or misdirection. For example, I suspect the warden that would probably have attended was "distracted" by the black council agents.