It was all about money.
The earliest Christians were poor, and as Christianity spread they gained more and more wealthy patrons who could finance the writing of Christianity. This process happened pretty quickly, because Paul himself was able to find wealthy supporters in Galatia, Corinth, and Rome between 50 and 65CE.
The Gospel traditions, memorized in parable form, were written shortly after Paul's death. Luke actually names Theophilus, who funded the writing of that Gospel, and he indicates that he uses earlier sources.