I think there's a few things at play here:

1) 'Leo' has always been presented as the charming "boy next door", this is primarily in the USA but heck, I'm in the UK and referring to him with a nickname.

2) He's avoided being involved in any major scandals, he seems pretty nice and genuine.

3) He has starred in a lot of top rate movies, often being seen as a very strong lead character. So much so he has a whole page dedicated to his awards and nominations

4) He has been nominated for the Oscars five times before this year, he was nineteen when he was first nominated. Again, he was Hollywoods teenage heartthrob. I believe he has attended lots of Academy Awards events, but there doesn't seem to be an easy to find list.

5) Leonardo DiCaprio is the subject of a few internet memes, the fact he had never won an oscar quickly became one in it's own right. Many people shared these memes with the subtext he had been hard done by and that Hollywood were purposely making him attend the Oscars just to deny him one.

I think that last one is the most important point, he's a likeable familiar face that got a lot of people rooting for him. I don't imagine the meme had much more life left in it but I feel the Academy were almost forced in to giving him an Oscar by public opinion. That's not to say I don't think he deserved it, I am glad won :).