"Why is it that the Catholic Church was founded on Peter and not Paul?" There is a little caveat to note before answering this question. The Catholic Church was founded by Jesus Christ since he is the foundation for our faith. What Jesus Christ did do, however, was entrust the Church into the hands of men so that his message would be taught and people be baptised through all nations and generations (Matthew 28:19-20). Jesus built the Church on men. The man who would lead the Church after its establishment would be St. Peter. How do we know this? Scripture tells us this; Jesus hand picks St. Peter himself: "And I say to thee: Thatthou art Peter; andupon this rockI will build my church, andthe gates of hellshall not prevail against it." - Matthew 16:18 (D-R) St. Paul's work for the Church was certainly monumental and he helped establish Christian communities throughout parts he travelled to in his time through his missionary work, but he was not the one hand picked by Christ to lead the Church, to shepherd the earthly flock (John 21:15-17). In a sense the Catholic Church is built on the work of the apostles and the Early Church Fathers, but Christ chose a man to lead his Church. This is not to say that St. Paul's work was not important; he certainly lead to an extent but not in the same manner that St. Peter was charged to.
Another point to keep in mind was that Paul's conversion took place some time after Christ appointed St. Peter to his role. Scholars suggest Paul's conversion between 33 to 36AD (after the death and resurrection of Christ; Acts 9) where as Peter was with Jesus throughout his ministry up until his death and resurrection. Suffice to say that event that is accounted in Matthew 16:18 took place between 30 to 33AD (the years of Christ's ministry). Paul was was Pharisee and persecuting Christians before his conversion.
* * * * * In summary: Jesus is the foundation of Christianity (there is no Christianity without Christ), but Christ entrusted the growth and mission of the church to men and appointed one man to be a shepherd to the flock in his name John 21:15-17. St. Peter is this man but this is not to say that the work of the other Christians during this time was any less important.