The way people are rewarded on steem is through something called the reward pool. The more upvotes you get, the larger amount of steem you will get from the reward pool after seven days.

Curation trails will upvote content based on how much you delegated to the curation trail. You don't have to create a good quality post to receive upvotes. Therefore, it is going to use up part of the reward pool.

When more people get upvotes on steem, the rewards are going to be smaller for all of them. This means that true content creators now will receive less rewards, because people are getting upvoted by a curation trail without merit.