Because the plot is very simple and the ending is not a message that people easily resonate with. My understanding is that Shinkai was trying to portray Hodaka bringing Hina back as "she was there for me so now it's my turn to be there for her", but it comes off as a stereotypical "power of boners / soulmates / etc". In addition, Hodaka's decision seems to affect no consequences for himself personally, so it's easy to find the resolution problematic (see the many comments including on this subreddit that say Hodaka is selfish).

I still enjoyed the movie more than Your Name, but it is the director's responsibility to make these above aspects convincing to the audience, and it seems not everyone came away with the message he was trying to tell.