Although they tried to move over assets from the 3DS iterations of Pokemon, a Famitsu interview confirms that they had to start over from scratch.
Amid errant concerns from fans that Game Freak just re-used 3DS assets, the developer dispels such notions, noting that they "rebuilt it all from scratch," all with a team of 143. The interview also confirms that Dynamax Pokemon have to be rebuilt from the ground up within the confines of the Switch edition itself, adding extra work.
With over 1,000 creatures in the mix at this point, crafting all of them again with a team of that size would be a daunting, almost MMO in scope project: hinting at why the National 'Dex is not in. The Famitsu talk also hints that the idea of bringing in everyone was "barely manageable" on 3DS, much less on Switch with even more original Sword & Shield Pokemon thrown in. As of now they are still undecided on whether or not to add more creatures in by way of a post-launch updat.e
As a concession, Game Freak says there will be a ton of content to go through, including the "Wild Areas," that double as junctions for raid-like boss encounters. I'm game! Although the National 'Dex completion was always my number one goal, I'll gladly settle for Galar region completion.