There was only a little time.  Soon Jesus would be tried and crucified (Mark 8:31): His earthly ministry would soon be over; and the trainee apostles had to be ready to take over as messengers of the gospel.  The first unsupervised assignment was as much, if not more, for their benefit as for the people.  In targeting their mission to Jews only, Jesus was removing the opportunity for cross-cultural problems to get in the way and potentially discourage the men.
 
They were sent to villages where Israelites lived; not Gentiles nor Samaritans.  Every Jewish male had enough understanding of the law to recognise the Messianic signs of healing and casting out demons (Isaiah 61:1-3).  So when the apostles came into their towns, empowered to heal people, they knew that the Messiah was soon to come.  The twelve clearly preached that He was near them and wanted to give to the crowds from His grace.  It was great training for the trainee apostles in preparation for their role at the start of the Early Church (Acts 3:1-10).
 
In case they were tempted to become proud, Jesus reminded them that the power to heal, cleanse, exorcise and raise people from the dead did not come from them.  It came from Him.  Indeed, the whole exercise was not intended to make heroes of the disciples but to point people to Jesus Christ and let them put their trust in Him (Luke 10:17-20).  The disciples needed to know that they had freely received power, which they could never generate, so that they could fulfil Jesus’ commission - acting and speaking in His Name (Acts 3:16).
 
Pride comes easily to anybody, and it may be a particular temptation to people who God uses.  It is so easy to think that our abilities can make the difference for other people.  That is a lie: we can only be of any use because the Lord has freely given us love, forgiveness, restoration - and spiritual gifts.  So instead of thinking how we might make a name for ourselves or increase our income (as Judas did), we should remember the privilege of being in Christ and being able to serve in His Kingdom.  Today, practise giving freely to others something of what the Lord has given to you.  That is the way He trains His servants.