Why do some wildflowers bloom before the trees even emerge?

A tree canopy in full leaf takes all the light away form the Forrest floor where the bluebells grow and as all plants need light to grow the bluebells must therefore grow and flower before this happens.

One could say why don't they leave it until after the leaves have come off the trees in autumn? If they did this then they would be starting to grow and flower in winter when the temperatures and weather are inclement and the amount of sunlight is falling to its minimum as the sun moves south.