Complex sentence: By late 19th century hundreds of "Lives of Jesus" were written, some of which were purely sensational and were produced not because any new data had appeared, but because the gospels were being read and interpreted in new ways.
Split into simpler sentences: By late 19th century, hundreds of "Lives of Jesus" were written. They were not produced because any new data had appeared, but because some people read and interpreted the gospels in new ways.