for years McAfee and Symantec pushed out bloatware that made people extremely interested in alternatives. The bloatware has been fixed, but there is so much free competition out there now!
Have you seen the prices for antivirus?!?!! $70+ for an antivirus suite? seriously? For something that is often beaten by free services? Even malwarebytes is $25 FOR LIFE, not per year, and that is a great one! If they sold their services for much cheaper they might stand a chance at gaining some market share again.
Win7/8/OSX/iOS/WP7/8/Android are all relatively secure out of the box now. It is not the Pre XP-SP2 days where just hooking up to the internet would get you a virus. You typically have to either click on something, or be specifically targeted in order to get an infection. On top of that web browsers tend to have warnings for bad sites/links, ISPs are filtering more and more content, and home routers are getting pretty good out of the box (and are super secure if you flash them with tomato or DD-WRT). Plus you have the major push for app stores which filter installable content, and a ton of pressure for applicaiton and web service providers to ensure that their product is relatively secure to begin with.
And also the largest sources of viruses was from illegally downloading music and movies which were infected. Now with so many legal free/cheap/affordable web sources this is nowhere near the issue that it use to be. And even some torrent sites are monitoring torrents to boot users that are infecting the pool.
Add to all of that Intel itself, who purchased McAfee specifically to implement security on a hardware level in order to better virturalize and compartmentalize environments so that the antivirus would eventually become irrelevant. McAfee's own work will undo their main product line! which is wierd to think about
There are just so many levels of security now, that it renders most use-cases for antivirus as obsolete today. It is not 100% secure or anything, and the internet is still a very scary place in some regards, but todays issues are on a higher level of security (with businesses and web services), and there is less and less focus on end-users as targets as their systems become more secure, and the users generally become more educated.