Why do birds eat nectar from only certain flowers?

Birds need the enzyme sucrase in their bodies, in order to digest the sucrose of nectar. And most simply don't have enough. Scientists think birds that can readily digest sugar, like warblers, have an adaptive advantage

Birds with longer, thinner bills, such as hummingbirds, bananaquits, and sunbirds, insert their bills into the flower so they can lick at the nectar directly ... Nectar-eating birds are called nectivorous if nectar does make up a large proportion of the majority of their diet, such as with hummingbirds.