The reason you don't get curation if you vote too late is because your vote isn't big enough to get curation.

So your upvote is worth less than a cent, but if that's lower than 0.003 steem you're below the dust limit so you couldn't get curation. (25% of 0.003 is 0.000x but steem only has three decimal places so you get nothing)

The reason you get the most curation at ~30mins is because you get a higher curation the more it makes after you've voted. That's why you'll often notice lots of smaller votes come in before a whale votes. But if you vote too early, you get less curation and are essentially donating your curation to the author.

So it makes sense for minnows to vote at some point in the first 30 mins, and whales to vote at any point after 30 mins.

But most last minute upvotes that are abusive are done by the same user using separate accounts, or a pair of friends hoping to cash out on lots of SP, so curation doesn't really matter to them - it's more important for them to do it late so people don't notice and flag it.