Because bees and wasps have short tongues, and need easier access to the nectar. These flowers also have landing platforms, and usually have a sweet fragrance. Bees and wasps prefer white flowers, or brightly-colored yellow, orange, blue and violet flowers. They are not attracted to red flowers because many insects can see the color red. Moths feed at night, and are therefore most attracted to white or pale-colored flowers that are more visible at night.
Moths also prefer flowers with a strong, sweet smell.
Many insects also prefer flowers that have nectar guides on the flower petals. These guides lead the insect to the source of the nectar, and different flower have very distinctive nectar guides. Many of these guides can only be seen under an ultraviolet light, a wavelength of light that can be seen by insect. Hummingbirds are the most common bird pollinators. They choose large, tube-shaped flowers to match their long beaks.