Sad WOTC can't get over having to play second fiddle to piazo.
I don't think it's a WotC/Paizo thing per se -- though 4E/5E vs Pathfinder does seem to be an influence. Let's look at some history.
As recently as 2012, WotC had a large presence in the dealer room, selling Magic and D&D products and having art both in the dealer area and in different parts of the ICC. Even then, though, there were complaints from other vendors how WotC being at the con was cannibalizing their own sales -- the only places who didn't seem to mind were the Magic vendors who sold single cards rather than sets/boxes. From a D&D perspective, there was still some 4E product moving, but the Next playtest was in full swing in a location just outside the dealer area, while the Living Forgotten Realms 4E Organized Play campaign had already departed the Sagamore Ballroom and was being run out of a ballroom in the Marriott Downtown.
In 2013, WotC gave up their dealer room space, and moved into Hall D with a giant castle that surrounded their gaming space. Also, perhaps coincidentally, 2013 was the year Paizo became a co-sponsor and took over the Sagamore Ballroom, which had been largely empty for at least a year beforehand.
5E officially launched in 2014 and the Tyranny of Dragons season kicked off. Baldman Games ran the Organized Play campaign out of Hall D, and WotC's castle was still present, but fewer members of the D&D team were given leave to make the trip as WotC's presence continued to downsize. Some staffers made the trip on their own -- Jeremy Crawford was one, as he attended the LGBT gamer panel, as he typically does.
WotC dropped its co-sponsorship of GenCon prior to the 2015 show, and also stopped sending the castle or staff to GenCon that year as well. There was some scuttlebutt over this having to do with Indiana governor Mike Pence signing the state's Religious Freedom Restoration Act, but it's not clear that was the real reason, and in fact the absence made perfect sense given the reduction in WotC's presence over the previous two years. D&D and Magic were still represented, with Organized Play being run by third-parties, and other vendors happily selling WotC product on the show floor. In that sense, it makes even more sense that WotC would leave the con -- if their products are selling just as well, and people are still able to play the company's games in an 'official' setting, why bother going yourself?
As someone who's attended GenCon since 2008 and can remember doing the D&D Miniatures game and Dungeon Delve in the Sagamore Ballroom back-in-the-day, I do feel nostalgic for the days when I used to hang around that part of the con seemingly all day. But times change. I'm still looking forward to GenCon, whether I get to gawk at Mike Mearls or not.