Miners can do whatever they want, it's entirely up to them. They decide.

There are usually two reasons for this to happen:
1. Fees are low, and miners are trying to force people to pay more by skipping all transactions with low fees (with current high fees this is very unlikely).
2. The previous block was found shortly before this one, and miners don't want to risk orphaning their block, so they quickly complete it without transactions. Block 498218 was mined 10 seconds after block 498217, so I think this is the reason.

This makes me curious though: how long does it take the mining pool to select transactions to put into a new block? I would expect this to take only a few seconds, which makes "12.5BTC now" an odd choice if they can get "12.5BTC + 4BTC" a few seconds later.