Good ol' Victorian poetry is always reliable when it comes to following conventional rules and patterns. The title, "Who Has Seen the Wind?" which becomes the poem's refrain, isn't trying to pull the wool over our eyes in regards to what this poem is about. Even before the Victorian era, poets would often begin their works with the title. Check out some of the Romantics for a better idea.
So Rossetti wasn't looking to stir the controversial pot when she chose to use her title as the focal point for her poem. Plus, the title is the central question that the speaker keeps asking, so it makes sense that we'd see it so often. By the end, we're feeling pretty convinced that no one has or will ever see the wind. So besides the title being rather conventional, we also understand that this particular question really captures the mystery and elusive nature of the wind passing by.
There's also something timeless about the title since we know we'd be hard pressed to ever find someone who has seen the wind. The "who" can be anyone at any time considering that the wind doesn't operate according to borders. And like most famous nursery rhymes (most of which start with their titles) our speaker tries to keep her message applicable to all sorts of tots.