The companies you said do have a better knowledge of the gaming world. Because they are in the business of selling games. Now it could be argued that IGN is also experienced in selling games, but even then, don't forget that IGN, at least legally, is a news / journalism outlet. The conflict of interest potential is huge. What if they use IGN to promote the games the store is selling? Or reverse that and promote games on HB that IGN covered? How long before all humble bundle / store games start to get higher ratings? Or IGN stops reviewing non HB/store games?
Steam/EA/'other publisher' buys Humble => cannibalizing Humble. Valve would not allow Humble to exist, EA/'other publisher' would push other publishers to leave Humble. Either way, no Humble in the future.
Project Red uses GOG to promote the games they
are making: Witcher ? Gwent? nobody questions that.
Humble is also a game publisher, nobody questioned the confilict of interest when that happened.
Valve is also a publisher. When Half-life 3 comes out nobody will question their confilict of interest.
IGN, GameSpot, EuroGamer, RockPaperShotgun, Polygon, PC Gamer ... they all promote Humble already: 'get this on Humble Monthly', 'get this Humble package' and so on.
IGN competes with GameSpot, and what GameSpot covers so must IGN, and vice versa. They all copy each other.
IGN cannot afford to ignore covering games that its competitors are covering.
If anything the conflict of interest is greater with IGN's competitors, GameSpot/GiantBomb, Polygon etc. Will they
stop covering Humble bundles and monthly games?
If they wanted to destroy it, they would probably be able to without buying it. Valve could just impose code time restrictions and the key selling market would collapse in a day. And EA will probably find some reason to sue them out of business, again, if they wanted it gone.
There's nothing wrong with EA selling on Origin or CDPR selling on GOG. On the other hand, IGN doing undisclosed promotion of HB games? The FTC and other bodies have advertising guidelines for a reason.
IGN can't afford to not cover those games, but their coverage will be marred by that conflict of interest. Whatever they say about a game, it will always be over the fact that they stand to benefit from being positive about a game on the humble store or bundles.