Darrin expands on a few reasons on the application of the Olivet Discourse. Click here to read it. (Though I differ with his point #2 on the interpretation of Matthew 24:14. I take this prophecy to be a Kingdom judgment announcement upon the ungodly world, which an angel will fulfill (Rev. 14:6-7), not the Church. This is unrelated to the great commission, which does not apply to Matthew 24:14. See this article for further explanation. However, the point against pretribulationism can still be made since Jesus in Matthew 28 is said to be with “you” (i.e. the Church) to the “end of the age,” and the end of the age has not ceased before the events in the Olivet Discourse (Matthew 24:14) therefore the Church age has not come to fruition before the events in the Olivet Discourse.)