Because the author says so in 2 chapters worth of background.
In more recent chapters, Takemichi was able to save Hina and when he goes back to the future everyone had realized that he was Takemitchi who traveled back to the past and had just came back to the future, without him mentioning it.
Although the story pleased me, the main point of the work is undoubtedly the characters.
Tokyo Revengers is a story with a spectacular premise… that is promptly squandered.
The same pattern is executed repeatedly, camouflaged by the addition of more characters coming out of nowhere with zero buildup but apparently all really strong and have some beef with each other to make it seem like there was a huge, complicated plot behind it all along, right from the beginning.
No-one is completely good or completely bad, and everyone is fallible to some degree.