The author is telling why people have started running as a sport a lot more. To know this, you can just say because this happened, this happened. Because people wanted to save money, they stopped going to gyms and started doing a cheap sport. The author develops the ideas by using the text structure by giving many reasons why people should/would start running as a sport. The 3rd sentence is a good example of the author telling why people would run. Hope this helps :)
