Why did the eleven apostles later become twelve again in Acts 1?
The eleven men later became twelve again because Peter saw the Old Testament program: after Christ was rejected, raised, ascended, then tribulation, then the King would return and set up His kingdom. The promise of this coming kingdom drove the eleven men to restore their number to twelve.
Les explains that Peter recognized the Old Testament program: Christ would be rejected, raised from the dead, ascend to the Father, then tribulation would come, and the King would return to set up His kingdom. This promise of the coming kingdom is what drove these eleven men later again to be twelve. In the surrounding Acts 1 context, Christ commanded them not to depart from Jerusalem but to wait for the promise of the Father, and He contrasted John's water baptism with the coming baptism with the Holy Ghost not many days hence.