The main reason Christ ascended to His Father in Heaven was so that the Holy Spirit would descend upon His Church on earth.
In John 16:7, Christ tells His apostles, "It is expedient to you that I go: for if I go not, the Paraclete will not come to you; but if I go, I will send Him to you."
This event, 40 days after Christ rose from the dead, ended His public ministry of imparting truth to the Church. The Apostles still needed, however, the grace of the Holy Spirit to put into practice the Great Commission Christ entrusted to them just before He ascended. In Matthew 28:19–20, Christ told them, "Going, therefore, teach ye all nations; baptizing them in the name of the Father and of the Son and of the Holy Ghost. Teaching them to observe all things whatsoever I have commanded you."
Because the Apostles did not yet have the grace needed to start this mission, Christ directed them to wait in Jerusalem until the Holy Ghost came. In Luke 49, Christ said, "I send the promise of the Father upon you: but stay you in the city till you be endued with power from on high." The Church thus began Her first novena as the Apostles anticipated the coming of the Holy Spirit, Who would ignite their mission at Pentecost.
Watch the panel discuss why Christ had to leave in The Download—The Ascension.