I haven't seen stats recently, but I know that as recently as a year or two ago, Yahoo had much more penetration internationally (ie, not US) than Google did. I'm not sure if that still holds, or if it applies to Brazil, but that may be part of it.

Google has almost 90% marketshare in Brazil. People love Google here (only this year Facebook overtook Orkut as the main social network), Chrome is also marketleader already.. Yahoo barely exists in Brazil, aside from a few emails.