Blame Dunga.
Ronaldinho left out of Brazil squad - FIFA World Cup 2010 - ESPN Soccernet
Long story short, Ronnie let his form dip over the last few years, until this year when he had a pretty good season. Dunga (the Brazilian coach) decided to stick with the players who had gotten the team through qualification & retain chemistry rather than bringing Ronaldinho back into the fold.
World Cup organizers chide Brazil for Ronaldinho omission