In the last steem update steem introduced this 12hr thing. Most people were seen upvoting content when close to payout time, so that they can get an instant payout. To solve this unfair voting techniques steem made a change.

We can vote and get paid in the last 12hr, but the voting stength reduce inversely with time.

12 hrs left - 100% curation reward
6hr left     - 50% curation reward
0hrs left   - 0% reward

I believe the problem was that people were voting a lot within this 12 hour period so that not as many people would notice and be able to call them out on it. If a whale constantly upvoted the same user in the last 10 minutes, almost no one would be able to catch them doing it. Same would apply to downvotes.

In the last 12 hours only downvotes are allowed, this is to prevent users to get big upvotes without a chance to revert them. 
Now we do have 12 hours where users can correct the abuse with downvotes.

This however doesn't prevent downvote abuse, this is why in the next version most likely the votes aren't prohibited completly, but are decreasing in value until the final payout. 

It is basically to prevent misuse of the site, the voting power, the vested power, stopping reward pool rape and selfish interest voting groups. An instance where whales even sought money for voting was found and the person involved had to admit of wrongdoing. In the above case, the whale had voted just before the last 12 hour period began and for a single user who had been paying a percentage. Such misuse is detrimental to the basic goals of steemit