Fan behavior + general meme/trendy-ness of it...
It isn't for any historical reason - I mean if you want to back to empire times, the French, Spanish, Dutch, Portuguese, don't get the same hate.. If you want to go for more modern examples, the German and Japanese don't get much hate despite their atrocities.
For me, it started with general fan behavior, the attitude of traveling fans, arrogance of media mismatching the performances etc. And then it just spirals to become the "trendy" thing to join in on.
The same thing happens with domestic football - right now Arsenal (even before they were the champions) have been the club to dislike for a while.. People cite fans, attitudes, behavior online, but never really any specific reasons, it just becomes the done thing, and people get on board.
--
Also for those saying that it is because people like to cheer for the underdog, so we will be rooted for more against Argentina... no chance! Argentina are so popular, world wide, all because of Messi - I don't live in the U.K. but instead a country that is not even in the world cup, and Argentina is a clear favorite here, and it is just because they love and know of Messi...
The only places that will be cheering for us next match will be Argentina's direct rivals, and the countries they just knocked out... I don't think we will have ever had such support coming from Egypt..