Probably fashion more than anything else. Feels like it's trying for the 'flat' design aesthetic that's become the norm online nowadays.
And I suspect that design was made to appeal to more 'casual' users, with the assumption being that a more minimalistic look would be more appealing/popular/usable for them.
But it's not really working. Based on people I've spoken to, many seem to despise the new design and almost everyone I've asked wanted to switch back to the classic version. That's not about techies either, it's more casual and often older users that seem annoyed by the new design too.