My understanding was the apostles were there to spread the good news to
the Jews - can't put new wine into old wine skins etc - hence the need to ensure
they were witnesses to what Jesus had done, his work his resurrection etc.
They had to be able to show the Jews that the promised Messiah was Jesus.
Still a difficult task even today.
They had a different role to Paul who preached to the Gentiles.
Incidentally both Paul and the apostles were extremely successful don't you
think.
Acts 1:20-26 NKJV
[20] "For it is written in the Book of Psalms: 'Let his dwelling place be desolate,
And let no one live in it'; and, 'Let another take his office.' [21] "
Therefore, of these
men who have accompanied us all the time that the Lord Jesus went in and out
among us, [22] beginning from the baptism of John to that day when He was
taken up from us, one of these must become a witness with us of His
resurrection." [23] And they proposed two: Joseph called Barsabas, who
was surnamed Justus, and Matthias. [24] And they prayed and said, "You, O
Lord, who know the hearts of all, show which of these two You have chosen
[25] to take part in this ministry and apostleship from which Judas by
transgression fell, that he might go to his own place." [26] And they cast
their lots, and the lot fell on Matthias. And he was numbered with the eleven
apostles.