Having a big delegation for @steemstem is connected to the visibility of the project. The curation rewards go to steemstem in this case, whereas to the users otherwise. Note that for users with a small SP number, I recommend trailing to ensure to have enough RC.
By sharing the curation rewards, we (try to) incentive the users to delegate and get something in return. By front-running everyone else with @steemstem, they may get smth similar (+/-).