I assume it's to make the group stage more exciting. As less teams qualify, more teams have important matches towards the end of the season. In NA, for example, matches like 100T-EG, C9-Guard, Sen-V1, etc. don't really have anything but prestige and seeding on the line.
So they had 6 teams qual instead. idk if it would be feasible, but they could've started the bottom seeds of each group in the lower bracket to make more matches matter.