I have been researching the historical reliability of the New Testament because I started having doubts about the Gospels. Fortunately I have found an abundant amount of evidence on the Gospels, and the historical evidence behind them. Some of the books that I have been reading are, "The Historical Reliability of the Gospels" by Craig Blomberg, "Reasons for Belief" by Dr. John Oakes, and "The Case for the Real Jesus"by Lee Strobel. However, there’s one answer to a question that kind of shakes my confidence in the Gospels. I have read time and time again that the gospels (Matthew, Mark, and John) were passed down by oral tradition. How do we know that the oral tradition passed down is authentic and eyewitness accounts? And why weren’t the gospels written earlier? There seems to be a gap between Jesus’ death A.D.30-33 and the first" gospel Mark 50-70 A.D. Is there any explanation to why they took so long to write the first gospel?
I am not sure I can give a definite answer as to why there was a significant amount of time between the gospel events and when the gospels themselves were written. One thing to bear in mind is that when scholars give dates for the writing of the gospels, they generally give the latest possible date. For example, scholars put Mark in the 60s or perhaps the 50s, but as far as I know, there is no way to rule out the possibility that it was written in the 40s or even earlier. So…. the numbers you see for the writing of the gospels of Mark and Matthew tends to be an estimate for the latest possible date, rather than the actual date. Luke and John are an exception to this. Given that Luke and Acts probably were written together, and given that Acts includes material from as late as AD 63, we know that this book was written later. Almost all scholars, including the most conservative ones, believe that John was written after Mark and Matthew.
So, as to Mark and Matthew, which are probably the first two written, let me address the question of oral tradition. It is clear that at least for a time, the gospel message was spread through the apostles and other inspired teachers. For the first few years, the gospel story was told as a first-hand or at most second-hand account of events which had happened in the region where the events happened. During this early period, there were enough eye-witnesses to the actual events that oral tradition could be corrected by feedback from those who heard the message preached. A number of studies of oral tradition have been done. These studies show that in a fairly closed society in which those hearing the tradition already know the tradition that is being spoken, the ability to pass on unchanged information is remarkable. Based on these studies, we can conclude that in the first twenty or so years, especially in the region of Palestine, a quite accurate oral tradition of the life and teachings of Jesus could be maintained.
Nevertheless, it is clear that as the gosple message spread significantly beyond Palestine and as time passed, the need for a written, and therefore unchanging, tradition of the life of Jesus arose. Mark and Matthew, and later Luke and John filled this need. The question arises: If, as evidence implies, the story of Jesus was carried by oral tradition for as much as twenty years, how can we trust as completely accurate the gospels which were written after twenty or so years? The answer is two-fold. Like already stated, studies show that oral tradition in a fairly closed society (such as among the mainly Jewish church in Judea) can be surprisingly well conserved. The second reason we can trust Mark and Matthew is that they were inspired writers. Like it says in 2 Peter 1:20-21, these writers were influenced by the Holy Spirit as they wrote. If we accept that God inspired these gospels, then accuracy issues are more or less dealt with.
I have addressed the problem of reliability, but have not really answered the part of your question about why the fairly long time gap before the first gospel was written. My answer is that we do not know for sure that there was not a written gospel account already in the 30s. Lack of clear evidence for a written gospel in the 30s is not proof that there was none. However, if we assume that the first written apostolic account was not until the 50s (a thesis that I do not assume at all, but which is believable), then the answer is that I am not really sure why God waited so long before putting in on the heart of Mark or Matthew to write down their accounts of the life of Jesus. Perhaps with the number of inspired teachers no one felt the need.