It could be any number of things, but regardless I think this is an important and empowering question to ask for a few reasons in particular. For one, it helps you decide whether some things are necessary. For example, if you think it is all the fun theme nights you do and really your students are just there for the relationships and connections then you could save yourself some time and money and be able to focus those nights towards other spiritual growth opportunities. This question can also help you see what you are doing well so you can both focus in on those areas and promote them more as well as seeing what you need to work on. For example, if they keep coming back because of your leaders then obviously they are doing a great job and you have recruited the right people and trained them well. So you will want to continue that because it is working and this is obviously what is keeping your ministry growing.
But the last reason why this question is good to ask might be the most important and yet the hardest to accept. This question can help you understand whether what you are doing is moving towards life change through a relationship with Jesus Christ or just giving them a cool place to hang out. Now, I have nothing against students having a safe, fun place to enjoy themselves. I also have nothing against high attendance and know it can be a good gauge as far as the health of our ministries. Not to mention the pressure many of us are under to make sure are numbers are above some line on some chart. But in the end we do not give an account for how we did on a chart but for how effectively we helped others meet Jesus. To further understand this you can look at stories like the rich young ruler. He comes and is excited about following Jesus. But Jesus knows His heart is not fully in it and the man ends up walking away. This story always fascinates me, because I'm thinking, "Jesus you had Him. Another point for the good guys." But Jesus knew that there was a greater purpose at work and He did not need half-hearted followers. I think we can learn from this. I say find new and exciting ways to bring students in. But if somehow students are continually coming back just for that then it may be that those things are that are attracting them to your ministry are actually distracting them from meeting Jesus. And may need to be done away with, even at the risk of a few students walking away for a little while. Because while it is always sad when any student leaves, it may be worth it so you can better minister to the students who stay so they can experience life change in Jesus in such a way that they can go out and help those who left experience that life change as well.