Because they released such a buggy and inferior product that's been responsible for millions of computers becoming infested worldwide.
...... oh, that's right, they haven't. A few thousand people picked up some malware when installing bootleg software and suddenly the sky is falling.
For the sky to fall, Apple would have to be Microsoft. So we Mac users have viruses and trojans to worry about, but in the context of that compared to Windows, our problems are low on the radar right now.
There really is nothing terrible about buying Symantec software. In older Macs, I noticed it slowing down my system, but now the faster processors, RAM, and hard drives make me not notice any drawbacks of some security software. There will be those who insist Symantec "planted" this into the Mac world to boost sales, but what is far more likely is that there are people out there who finally decided we are a good enough target to shoot for.
I think over the past few years, iPod and iPhone has made Apple more visible in popular culture, and thus OS X. In a way, we have hit the big time and are a player in the computer market, and the bad guys will want to take us down putting us in the same league as a Microsoft.