Because it's something that is hoped for in the future but is by no means certain, so it is a subjective desire. At least that's probably what the correct answer will be, but I think that the real main purpose of the subjunctive is to confound students of French, so you will often find it in places you don't expect to, and conversely it will often be missing when you do expect it. Part of the fun and challenge of learning a language! 

un avenir qui nous plaise -- A future which may please us.

This rendition in English shows that you're actually expressing a wish in French.