Because there will be vast swathes of PS3 owners who are still completely clueless and will only be aware of things once emails arrive.
I didn't say it should be kept solely to emails but I disagree that forums and gaming websites are able to 'reach far further' than email as you originally suggested. They'll reach a limited audience (ie. gamers who read gaming sites and forums) far far faster but I don't agree that they will reach further.
World of Warcraft is a good example.
About 90% of the guild I am in didnt know there was a patch today
About 75% of them have no clue what changes the patch will bring
Despite patch notes and changes being posted on wow sites for the past 3 months! Out of the 77 million accounts that have been stolen the vast majority probably still have no clue as to why they cant logon to PSN let alone someone has stolen their details