*Gamefreak
There are 66 million 3DS consoles sold.
The Pokémon models were already future-proofed for HD use, so that's not the issue.They only did this because TPC puts in bare minimum effort on the iterations and they literally don't have to make any new assets as long as it's on the 3DS.
But the biggest install base would be putting it on both.Install base
Nintendo likes money