Why was a top secret the Zimmerman telegram sent by the Nazis to Mexican President Carranza ?

First of all, it was not the Nazis. The Nazi (National Socialist Party) was not created until after the war. The Germans knew that unrestricted submarine warfare would likely draw the United States into the war (which it did in April of that year). The Germans hoped that by getting Mexico involved, it would distract the United States and force the U.S. to send troops and ships to deal with Mexico instead of fighting Germany. It is likely that this was not even a realistic proposal and that the telegram was meant to be intercepted. The hope would be to sew seeds of mistrust and cause the U.S. to focus some of its attention on Mexico instead of on Germany.