Why would hawk moths evolve long tongues for Darwin's Star Orchid when there are other flowers around?
I think the key terms are niche and co-evolution;

Why would hawk moths evolve long tongues for Darwin's Star Orchid when
there are other flowers around [?]

Because the moths have grown a long proboscis as an adaptation to a certain niche. As you say, not much other species can feast on the flower's nectar, so it makes perfect sense to narrow your niche by elongating your proboscis, and reap the benefits of this little visited flower.
A niche is

[T]he range of environmental conditions in which a species can exist.

As the University of South Africa explains:

One important part of the idea of the niche is that it is controlled
by how the species exploits resources in its environment.  If there
are two species living together and exploiting the same resources,
then one will usually be worse than the other at doing this –
eventually, that species will disappear because the other species will
outcompete it. However, species can live together because they
exploit different resources or because they exploit the same resources
in different ways.  These species then have different niches, even
though they occur in the same place.

Then how would the flower benefit? Now I'm going a bit more into a hypothetical explanation, but flowers need cross-pollination to reproduce and genetic mixing. If a plant species can bind a certain pollinating species to itself, it can make sure that that pollinating species only forages on that particular species of plant. Hence, that plant species can ascertain that the pollinating species carries only the right pollen and therefore make sure the transferred pollen is from the same plant species. This is beneficial, as pollen from other species won't pollinate them.
It's all a matter of co-evolution, in this case of two mutualistic species, nicely and succinctly explained by the University of California, Berkeley:

The term coevolution is used to describe cases where two (or more) species reciprocally affect each other's evolution. So for example, an evolutionary change in the morphology of a plant, might affect the morphology of an herbivore that eats the plant, which in turn might affect the evolution of the plant, which might affect the evolution of the herbivore...and so on.
Coevolution is likely to happen when different species have close
ecological interactions with one another. These ecological
relationships include:

Predator/prey and parasite/host
Competitive species
Mutualistic species



The hawk moth Xanthopan morganii praedicta pollinates the Madagascar star orchid (Angraecum sesquipedale) with its long proboscis. source: Encyclopedia Briattanica