Thanks - really good questions: and to be honest there are a lot of answers and moving parts to this: so answer is not limited to these potential reasons:

1. A lot of people coming into the tag are very very small minnows, so the current support (which is passionate and supportive experienced Steemians with Steem Power) just won't cover it. Delegation is required to provide more adequate cover for the excellent grass-roots work.
2. A lot of Steemians in the group are in "developing countries" using their earnings for a living and so don't power up. So they may not grow to contribute any significant upvotes themselves. There is a humanitarian angle to the community here and delegation will help with this.
3. The time period covers several weeks when currency has been falling so Author rewards have fallen as a result too.
4. Some poor quality posts which might be good intentioned but appear spammy come into the tag probably as a result of seeing early rewards from being involved in the promo-steem tag. The new CAMPUS system and SteemAmbassador Curation could help minimise this.
   Thanks for the Q.