You know... I honestly don't know. I think it has to do with both trading and the ability to play it on the go. Which is cool for the handheld. But I don't see what that had anything to do with consoles. The main reason was most likely because since handhelds are where Nintendo dominates, that the Pokemon Co. kept them there in order to keep high sales figures.
Hardware Comparison Threads:
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PlayStation/Xbox/Switch: 2022 Edition
PlayStation/Xbox/Switch Hardware Battle: 2021 Edition!
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020
PlayStation 4/Xbox One/Nintendo Switch: 2018 vs. 2019
PlayStation 4/Xbox One/Nintendo Switch: 2017 vs. 2018
PlayStation 4: 2015 vs. 2016 vs. 2017