Holiday is younger, bigger, can shoot, has two healthy knees, and just gone done with a season that is, at least in terms of traditional statistics, better than any regular season Rondo's had.
From the Pelicans' perspective, that means Holiday is more valuable than Rondo.
I'm not saying that's the correct way to compare the two players, but I imagine that's the line of thinking. As somebody else pointed out, the Hornets / Pelicans have already shown they value young scorers over an All-Star pass-first point guard like Rondo.