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Because the pokéwalker's creation was borne out of a middle-aged man on a fitness kick, who wondered about creating a pedometer that could carry a pokémon.Something so primitive could be sold and appreciated by the target audience back then, but in the here-and-now when kids either have their own phones or access to a phone of some description, it's not going to work (this is what Masuda meant when he said he was competing with phones for attention).
It's worth noting the last accessory to the games ended up being a controller to work with one game to give an actual reason to buy it, as well as acting as pokéwalker for three (one of which requires people to get out of their living quarters and explore the outside to meet pokémon, like people needed to take steps to charge watts in the walker to meet new shit in that).
If some gimmicky toy idea crops up, that they know kids everywhere will cut a cunt to get their hands on, then yes, we'll see some twee shite pop back up alongside the games. but it seems the world has moved on from where they were at in 2009 and that doesn't look like changing.