Matthew Chapter 2

2:1. When Jesus therefore was born in Bethlehem of Juda , in the days of king Herod , behold, there came wise men from the East to Jerusalem VRSEa_mat_2:1>, VRSEb_mat_2:1>
Cum ergo natus esset Iesus in Bethleem Iudaeae in diebus Herodis regis ecce magi ab oriente venerunt Hierosolymam VRSEc_mat_2:1>

2:2. Saying: Where is he that is born king of the Jews ? For we have seen his star in the East, and are come to adore him.VRSEa_mat_2:2> VRSEb_mat_2:2>
Dicentes ubi est qui natus est rex Iudaeorum vidimus enim stellam eius in oriente et venimus adorare eum VRSEc_mat_2:2>

2:3. And hearing this, was troubled, and all with him.VRSEa_mat_2:3> VRSEb_mat_2:3>
Audiens autem Herodes rex turbatus est et omnis Hierosolyma cum illo VRSEc_mat_2:3> 2:4. And assembling together all the chief priests and the scribes of the people, he inquired of them where Christ VRSEa_mat_2:4> should be born. VRSEb_mat_2:4>
Et congregans omnes principes sacerdotum et scribas populi sciscitabatur ab eis ubi Christus nasceretur

Why has it written 'When Jesus therefore was born in Bethlem of Juada, in the days of King Herod......

The word 'therefore' is not necessary.
What do you think?
Since "therefore" indicates an immediate consequence with what precedes, well, no, there's no reason why it should be there. "Thereupon" could do, though.
Most English translations have either 'Now when Jesus was born' or 'After Jesus was born'.

Cum ergo, in Latin, literally translates into English as 'when therefore'. The verse does succeed an account, in Matthew, chapter 1, of the lineage of Jesus, and the details of his birth, so it doesn't sound terribly odd to see that detailed account followed by 'when therefore', as it means 'consequently to Jesus having been born, there came wise men from the East'.