Maybe it comes from Italian, or maybe it's just our way of changing words in the spoken language. Just like in English, when they say 'somethin', 'doin', or 'gotta', 'gonna' etc. I think this happens in every language. We simply find better versions to say some words. You can also read this post. It's quite funny. :D http://cinabru.blogspot.ro/2007/08/eu-sau-io_7178.html

bad habit :) .... its not a formal word (language)....

"io" is an informal version for "eu". Please note than even if you say "eu" , it is pronounced actually "ieu" (it's difficult to pronounce otherwise). The same happens for "este"=is (pronounced "ieste") or the short "e" (pronounced "ie").

Cause hip-hop, man! Okay, seriously, for the same reason English speakers say 'yeah' or 'yep' instead of 'yes'. Now another question you might ask yourself is why they write 'eu' something that they actually pronounce [je͡u ], and therefore should rather write 'ieu'. Just an exception, I guess.