If there's one thing the Internet is good at, it's keeping track money and knowing where it has moved.

Ransomware attacks typically request payment for a decryption key. How is it with such an easily tracked transaction that authors of ransomware aren't prosecuted more often?

They use alternative methods from traditional credit card/wire transactions, namely prepaid cards and crypto-currency like bitcoin. This is because it's much easier to stay hidden this way. 

This article outlines more information if you're interested: TeslaCrypt: Following the Money Trail and Learning the Human Costs of Ransomware  
