This question was recently asked of me. Below is my basic answer without having too many details.
Question: Why don't other singles ministries (churches) promote events that other churches for doing for singles.
Answer; Some churches do feel you might steal their singles and as a result do not tell their singles about other events going on in other churches.
Answer: Some churches/pastors/leaders forget to promote due to their plate/calendar being full
Answer: Some churches are not as concerned for singles being connected as others.
Solution:  You have to build a relationship with each pastor/leader from each church so there is a mutual relationship and understanding of the vision of the ministry. If you simply, have an email agreement or surface relationship, then there is no buy-in or ownership of the city-wide ministry emphasis. The more you are connected to these other pastors/leaders, the more likely the will help get the word out, as well as be apart of the event. Also, think about rotating event locations to various churches as well as having the pastors/leaders of these churches be the host. Why invent a new event vs. supporting an existing event at one church.