It's because, despite what they promise each year, TSM plays to keep their stranglehold on NA and not for international success.
Their entire philosophy is centered around holding NA down by exploiting its specific meta but that leaves them with little or no ability to adapt to different styles.
It's no coincidence that, when TSM is not at an international event, NA tends to over perform expectations, as other teams are more able to adapt. Or why, for many regions that are not Korea, the second or third seeds tend to do better in general.
G2 over in EU had this problem their first few splits, but they seem to have corrected this while TSM just seems content to double down on the strat.