Because people should be able to vote naively on things that they like, and reward them just as much as if they were sophisticated. Every change like this makes "proof-of-brain" less about evaluating the value of posts and more about understanding the innards of the voting system.
For instance, people who know to wait fifteen minutes before voting good comments made on their posts will now advance faster than those who don't. That's pretty much purely stupid.
But this was introduced in order to equalize the chances between bots and manual curators. Otherwise, the bots would vote immediately, taking most of the curation rewards.
If someone finds a valuable post, it is usually more profitable (in terms of curation rewards) to vote much earlier (even in the 5th minute) than to wait until 30. The key is that the formula for the curation rewards takes into account also the sum of rshares before your vote so if it is low enough, you gain more from being before others than lose from voting before 30 minutes.
I pasted the link to my last post here: http://steemer.pl/curation/post-curation.html
And this is the result:
The best performance was achieved by the person who voted in 4th minute! :)