What would you rather to get for June?
Long overdue mastery 2.0
or
new content
The reason behind this pull is, to show Kabam that people are more interesting on mastery presets/overhaul (a system that all the community gonna like) vs a new content (which is great, but as soon as you complete it, you most likely won't do it again).
so they put more energy revamping it rather than delaying it further and further.