Why would a flower evolve such that a particular pollinator cannot pollinate it?
I think the explanation by the interpreter is most appropriate. The definition of species is a particular category of organisms capable of reproduction among themselves. Thus pollen of a particular species must fall on the stigma of the same species. But pollinators visit all species of flowers (as they never visit Biology Stock Exchange, thus ignorant of the definition of species) which bears nectar and the entrance is big enough to let them in. 

The flowers in your question increase their chance of pollination by reducing the varieties of flowers a particular pollinator can visit. Lets make it simpler (or even more difficult for those who don't like mathematics!)... 

Suppose there lives 4 humming birds and 4 bees in a oasis...both are too lazy to visit other oasis in search of nectar. One morning 4 flowers bloom in that oasis...2 wild flowers you mentioned and 2 wide mouthed flower of some other species.  Now 1 flower of the wild species can be visited by 4 bees (as humming birds can't enter the flower). One flower of the other species can be visited by 4 bees and 4 humming birds. This large number of pollinators for the second type of flower reduces its chance to be visited by a pollinator that visited a flower of the same species. Whereas for the wild type flower the possible number of pollinators are very few (only 4)...so the chance of one bee visiting two flowers of the same species is very common and so is the chance of it getting pollinated. 

In this way these flowers increase their chance of pollination by modification of their shape.