Tsubomi is part of the problem. She helped bully him for years too. I don't know why so many of you are trying to whitewash her involvement in all of this because she is also a virgin. That ain't no excuse for participating in bullying.
I think many people are not trying to hide or forget about Tsubomi bullying the main character, but because the beginning of the story shows that he should choose one of these three people because he is not very normal either.
Another reason is that she may not be the best girl but out of the three she is the least dangerous and many times we see her miss or get countered.
Another important reason I think is the "storytelling". Tsubomi's story seems to have a backstory that is waiting to be revealed and seems so separate from Chisaki and Kanano that when it comes to her part, I almost feel like I'm reading a different manga.
As I said, this story is full of strange people, even Moka seems untrustworthy, and judging from the way it's told, she might turn out to be the most dangerous, so out of all the options, we can say that Tsubomi is the safest. (To be honest, Tsubomi doesn't seem that smart. If she were the only one, I think she would be easily countered. When she and the main character are together, it feels like a pair of idiots trying to win against each other, and it's really funny.)