Honestly, I have no idea where this "No one went to the Seminars" thing came from other than wild guessing.
Mike Mearls himself said it. Can't remember if it was a tweet or from his Tome Show interview or what. He said they got many more people to attend D&D seminars at PAX than they did at GenCon, so they're focusing on PAX for seminars.
Which I definitely can understand if we're looking at it from the point of view of level of involvement in D&D plus the intensity level of said panel. And what I mean by that is this: the D&D panels I've seen at PAX East have been usually one of two types-- either it's the "hype" panel for the upcoming game/story, *or* it's the "Help me with my game!" Q&A panel for Dungeon Masters. In both of these cases, they have seen and felt like "introductory" type panels. Panels for people who don't already have a big hand in the game. The stuff in the hype panels were things I already knew about just from coming to the WotC site and EN World... and the DM-info Q&A was the panelists giving out tips that most of us "experienced" players already know or have heard before.
So why did these panels do so well at PAX? If I had to guess, it's because most PAX attendees are *not* D&D focused by any stretch. Or heck, not RPG focused by any stretch. So there are a lot more gamers out there for whom these types of introductory type panels might actually be interesting and useful. And considering that PAX does not have "continuous" gaming all weekend long (unless you sit in the personal game area just pulling games from the lending library and play with a couple friends), there is plenty of time to take in a panel for D&D that doesn't disrupt the rest of your weekend. Thus, they show up. But at GenCon? Perhaps these panels are just too "low-level" to get most of the D&D community to come over from the game tables to attend, and most of the non D&D or non-RPG players have enough other stuff to occupy their time to carve out space for a panel about a game they might not have any real interest in.
I think that's what they mean when they say that GenCon is for playing games, and PAX is for learning about games. At GenCon, there is always a game to play if you want, so a panel has to be *really* interesting for you to pull yourself away from the game tables to do so. But if the panels the D&D department for WotC can put together just really aren't all that in-depth or interesting to hardercore D&D players, why bother trying? I mean, sure, if they held back all their "upcoming information" of new products for a panel at GenCon, that panel would probably be packed to the gills. But does doing that, rather than giving out the information on their website/Twitter whenever it is ready, really garner them anything tangible in the long-run? I'm not so sure.