Acts 1:23

And they put forward two, Joseph called Barsabbas, who was surnamed Justus, and Matthias.

They put forward ... Who did the putting forward? Is it to be supposed that the 120 disciples mentioned a little later did this? There is no evidence whatever that such a group had been disciples from the beginning of John's baptism; and thus it is not reasonable to suppose that anyone participated in the selection of Justus and Matthias except the apostles. Furthermore, there is a strong inference in this passage that only two qualified men could be found, other than the apostles themselves. It appears that those two were equally qualified, hence the decision through casting lots.