It's sad that someone would hack a site like this. The only thing they could possibly want are the email and password combinations. I hope they are stored in a secure way.
There are plenty of other motivations. It could be:
1) a novice hacker looking for experience
2) an automated bot that scanned IPs for some vulnerability and acted on its own when it found one
3) someone looking to extort the owner for money
4) someone with a personal vendetta against the owner
5) someone looking to secretly plant bitcoin mining software on the servers (who got caught)
6) someone looking to alter their own score to help them get some sort of math job
I could probably go on for a while, but you get the point. It's hard to ascribe intent to this sort of thing without more information.
As Project Euler is completely free and has a positive impact on the programming community, it would be very sad if the reasons for the hacking were 3) or 4).
I would discard 1) cause a real hacker, even if novice, would enter without damaging or stopping the service. He/She would maybe report to the owner the vulnerability or even would keep it secret.
It was posted elsewhere that visiting the hacked site triggered downloads of "flash_updater.exe" type trojans, so maybe one reason for the hack was to spread malware.
Think about agencies, not private actors — then this site makes a perfect target. Recruit and subvert are basic tasks for these agenices, and to do so you need information about the candidates. This website had probably a significantly higher proportion of interesting prospects.
