I believe this was because @Ueda wanted diversity in mastery combinations in the player base instead of only roughly 24 mastery combinations (assuming the advanced mastery chart is accurate).
I do agree that there should be a way to know which mastery combination will give us advanced mastery but since the development team is still working on that and most probably the combinations are not yet determined so let's just be patient. Nonetheless, I believe that we should play with the mastery we like instead of what will get advanced mastery. For example, if you like to play as Earth/Med, why should you get a mastery reset to solo med, solo earth or other advanced mastery combinations that you don't like that much just because of the advanced mastery?