I could come up with a number of reasons, mainly not wanting to give up their (stated) mission of humanitarian aid. I'm sure reasons you could come with would be much more sinister.
There's no evidence to support that Hamas organized this or that their intention was to bait Israel into action. After all, the victims were mainly foreign citizens, and on board were multiple relatively famous political activists including the swedish writer Henning Mankell.
Ah yes, the age old "bitch made me do it!" excuse. Sorry, won't hold up in court.