As for the advertising angle....
Is there a reasonable chance that significant numbers of folks going to brick-and-mortar stores for Free RPG Day *don't* know that D&D 5e exists? If not, then it isn't really a major advertising opportunity, in general. D&D doesn't need a general name-recognition boost with this population - it seems to me that they are among the most likely to have already bought a D&D product recently. Wouldn't WotC's advertising dollars be better spent reaching folks who probably haven't bought recently?
As for the support angle...
There's an argument that the game could use more support, sure. But is one small product on Free RPG Day going to be a major element in addressing that? If you aren't satisfied with their support, wouldn't you rather see them spend their effort on something more substantial?
Free RPG day is great for smaller products, or for games that have a hard time getting the word out about their existence. D&D isn't that kind of game.