The Bible says that “No good thing will I withhold” and “He will give us the desires of our heart.” That being said, if we pray for someone’s salvation, but they are not one of the Calvinisticly elect, then God cannot keep His promise. Perhaps some Calvinists believe that we should never pray indiscriminately that the lost become saved, or else we might defy God’s sovereignty. After all, if we don’t know who God has Calvinisticly chosen, and we can’t know since it’s a “secret,” then to be on the safe side, we ought only to pray that those that He has elected become saved, which of course will happen anyway.