Christianity was always intended by God to be a ‘movement’ – spreading out in ever widening circles from Jerusalem to Judah then into Samaria and continuing throughout the whole earth!
In Acts 9 we see Peter going on a missionary trip without John – at this point John’s name drops from the record and Luke records only the movements of Peter. After Acts 12 Peter’s name in only mentioned once (Acts 15 – where he recounts what God did through him in the past). From this point on Luke turns his attention to Paul and continues with Paul until the end of his account!
The answer as to why is quite simple – Luke was documenting the advance of the world Christian movement – Paul was a “regions beyond’ apostle and was at the forefront of Christianity’s advance up until the time Luke concluded his writings and his treaties to Theophilus (Acts 1:1).