WHY did Tsm have to participate in a tournament to get to MSI

It goes off of past performance at international events per region. If you check the esports page and go to MSI they break down their way of determining this.

Basically the reason is that the stronger regions (regions who have performed better at recent international tournaments) go straight to the group stage or, in TSM's case the case, the second round of the play-in stage. Likewise, weaker teams have a bit more to do to get in.

FW also had to fight for their spot in the main event as well. As Paquay stated, there's an article that breaks down the reason this time.

cause they not even close 2 top tier

Only 3 of the 13 teams got byes. At least TSM got a partial bye.

I guess if you're going to have a totally odd number of teams, some of em are going to have to get a bye. But the only team that deserved one is SKT.