The amount of views for each photo in a challenge is dynamic and changes according to the number of participants and voters.
The Exposure algorithm is designed to give every image equal opportunity to get voted on.
A new photo gets exposure fast because the voting system strives to equal out the views across all photos in challenges.
Once you reach the top amount of views your exposure will slow down but at the end of the challenge, all photos in the challenge will have a similar amount of views.

All the exposure algorithm does is to make sure everyone gets the same amount of views. 

The reason the exposure of your photos slows down is BECAUSE of fairness to other photos that haven't reached that amount yet.
Each photo's exposure will start slowing down once it reaches the top amount of views available per photo in that challenge. 

The whole algorithm is built to give all photos that participate in the challenge the same amount of exposure.
What you are experiencing is the system trying to give other players the same chance as you have.