I think players choose the half-orc for two reasons. First, they choose the half-orc because they want to roleplay. They play the character either as an outsider, someone who could either be a noble savage wrongly persecuted by the prejudiced, or as a dark-inclined character. Second, after 3E, they began choosing half-orcs because of the +2 Strength bonus. All of these reasons for choosing the half-orc are covered by other classes in core 4E. Outsider/dark character? Meet the Tiefling. Strong character? Meet the Dragonborn. I am not sure that i like the new races much, but I understand that the main reasons I favor the half-orc is traditional flavor and not mechanical. I remember the half-orc from first edition and have gotten used to him, whereas I can't quite picture towns with Tiefling bartenders yet.
Marnak