It is so that your voting power does not decay in a linear fashion otherwise your vote power would hit 0% after 50 votes (50 x 2% = 100%). It ensures your vote is always worth something and you can vote as much as you want - just with diminishing returns.

Here is a good article that talks about it :-
https://steemit.com/steem/@vcelier/should-you-care-about-your-voting-power