It seems that the success of the Steam gaming platform may not be entirely fair for other sites - the developers were sued because of their pricing policy. Recall that Valve (the owners of Steam) insist on the same price for all platforms, which is significantly different from the policy of other sites selling games.
The lawsuit states that such actions on the part of Valve "undermine the competition in the market", creating unfavorable conditions for other sites, including the Epic Games Store, Microsoft Store and others. In addition, lawyers called Valve's actions "abuse" and indicated that such actions create obstacles to healthy competition in the market. Recall that today Steam is the largest platform for the sale of games and the authors of the lawsuit believe that Valve seeks to "absorb" all ties between creators and players.
Interestingly, this is not the first lawsuit against Valve in recent years - more recently, the developers were sued for the possible theft of a patent for the creation of controllers.