The illusion here is unusual because it is rare we fix our attention on any one thing with quite so much attention. The image is also blurry and already quite faded, which predisposes us to this sort of mind-bending sorcery.
Selective processing is perhaps more noticeable when it comes to the other senses. It is why you rarely smell your perfume a minute or two after application and you aren't constantly aware of the feel of your clothes on your skin.
Interestingly, it may be same phenomena behind the Bloody Mary mirror illusion. A 2010 study published in the journal Perception had volunteers stare into a mirror in low lighting for 10 minutes. Sixty-six percent reported seeing major facial deformations, 28 percent an unknown person lurking behind them, and 48 percent perceived mythical and monstrous creatures – course, it's all just the mind playing tricks.
There are several other images that can also induce Troxler's effect, including a circle surrounded by a bigger circle, a slightly sinister "Cheshire Cat", and a Lionel Richie lookalike surrounded by a circle of purple dots.
[H/T: The Verge]