You forgot number 4: the Vote Bombing Campaign.
I really don't think that this is one of the major reasons.
With its deceptive description the manga attracted many people who expected a far better story. Sure, there are plenty of mangas out there that are probably worse, but the thing is that many readers don't vote for these, because they didn't start reading them due to their premise. This manga had an interesting idea but the author destroyed the story from the beginning.
Instead of focusing on the original idea he added a lot of unnecessary drama. Wouldn't it have been enough to banish Est? Does he have to be in love with the person banishing him? And what's with all the hints that Daru and Athen will become lovers (even if they won't till axe) and the misunderstandings due to the lack of communication? Just let Est leave the story and use the time wasted with the drama to show the motives for their actions properly and some real growth of the characters (not shopping trips).
The author abandoned the original idea to create a story centered around a whole party (with the FL) growing in favour of a story about Daru, the most unlikable character of the story (due to all the unnecessary drama). All you get to see is a character who was strong from the beginning and decides to show his powers at the worst point possible (after banishing the supporter). The rest are only episodes to justify all his actions which don't make sense ("Hey man, I couldn't clear up the misunderstanding in the guild or afterwards in private. It's better if she thinks that you are some arrogant, vile person who shows his true colours after becoming powerful")
If the description had been more truthful and started like "This is a story about an old drunkard with great powers who decides to reveal his strength after banishing the supporter, who is in love with the party leader, because of ... reasons..." the manga would probably have better ratings because most people wouldn't have even considered reading this crap thus not rated it low. The readers were clearly deceived and acted accordingly.
