I'm still believing the main reason why it looks better is because the first trailer was rushed out and the bitrate of a stream made the quality of the game look worse than it should even post-stream (2-3 frame Chingling was born because of it). It's still looking alright for the most part, but...
Posting Joe Merrick's tweets here because there's more tidbits that being shown when looking at the game deeper.
English/Korean trailer differences
Wurmple difference
Wurmple in Legends Arceus:
Wurmple in every game prior:
Could mean that Wurmple or at least the Beautifly and Dustox lines...