Nerfs affect the meta and balance much more.
But buffs allow for more options, and considering Specialist is their new format, it makes sense for them to buff cards to try to give as much options as possible for all the classes.
Only nerfing means that the weak cards remain weak, and now there may be even more weak cards. Only buffing could make everything look OP and the game down to "who draws their stuff first". But coupling nerfs with buffs, more options should appear.
1