the trackers are after kino because of the pearl he possess, also because of murder of one of the men who attacked him. if the trackers catch kino then they will also be paid a high amount of money. so greed is the main thing making the trackers pursue kino.
The notion of greed and its destructive nature are very evident in most of Kino's decisions and of the consequent bad turn of events for him as the story progresses. The men are after Kino because he managed to kill one of the trackers earlier in the chapters. They are out to revenge their friend, but more than than, greed, their desire for the pearl, makes them chase Kino.