Because they did not give the children of Israel bread and water when they came to them, but got Balaam to put a curse on them: though the curse was turned into a blessing by our God.
See, the people who have come out of Egypt are covering all the earth: now, put a curse on this people for me, so that I may be able to make war on them, driving them out of the land.
For I will give you a place of very great honour, and whatever you say to me I will do; so come, in answer to my prayer, and put a curse on this people.
Then Balak was full of wrath against Balaam, and angrily waving his hands he said to Balaam, I sent for you so that those who are against me might be cursed, but now, see, three times you have given them a blessing.