What two reasons did Elizabeth give for her conviction that Wickham would not have marry Lydia?

Elizabeth believed that Wickham would not marry Lydia because they hadn't gone onto the Gretna Green (where they would have been married), and that Denny had told them he didn't believe Wickham had ever intended to do so. 

Though Lydia’s short letter to Mrs. F. gave them to understand that they were going to Gretna Green, something was dropped by Denny expressing his belief that W. never intended to go there, or to marry Lydia at all, which was repeated to Colonel F., who, instantly taking the alarm, set off from B. intending to trace their route. He did trace them easily to Clapham, but no further; for on entering that place, they removed into a hackney coach, and dismissed the chaise that brought them from Epsom. All that is known after this is, that they were seen to continue the London road.