You will probably find that the attack was directed not at PPrune as such but at the software the board was programmed in.
PPrune is - if I am correct - a vBulletin board written in php/sql.
SQL has had a hard time of late with a concerted attack by hackers to force errors in applications and allow an unauthorised user to gain access to the admin functions.
Some of the electronics forum that I subscribe to use phpBB, another php/sql board, and have been attacked in a similar manner. One had 6GB wiped off the drive.
Once the hacker finds a way to exploit what is generally an untested error then they just troll the web looking for boards to attack. After the first one has been done then someone is working on a fix but that can take a bit of time.
The short answer as to why is... pot luck.