It's because it was submitted for the Emmys in the Limited Series category, while "The Falcon and the Winter Soldier" was submitted as an ongoing.
I don't know if it means much though. Either of these shows are likely to have different titles if/when they get a second season. "WandaVision" is a title specific to the television sitcom format, and while Wanda and Vision may get another series, it's more likely to be "Vision and the Scarlet Witch" or something like that. And Sam is unlikely to go back to being the Falcon.