Don’t overthink: the simple solution is often the right one
Charles Darwin was quite popular: he even got flowers! From a botanist, that is. For research. But still! He got flowers from the Comet orchid or Angraecum sesquipedale
native to Madagascar. He didn't put them in a vase or dried them to accessorize. He cut them open and examined them. And What He Discovered Shocked The World!!1! ; -)
Botanist James Bateman send Darwin flowers.
From Darwns book "On the various contrivances by which British and foreign orchids are fertilised by insects"
Orchids
Before we continue with this Shocking Revelation, it's good to know orchids often get pollinated by insects. Not just bees, but moths as well, which lick the nectar with their tongue and simultaniously transfer pollen.
Bee orchid. Ironically, this orchid isn't pollinated by bees.
A weird moth? Of God?
When Darwin inspected the flowers, he found out the nectar of this flower was about 30cm (11inch) deep into the flower!! So Darwin drew a simple solution: somewhere on that island was a moth with a 30 cm long tongue.
Excuse me? Can you imagine? A little moth with a tongue 10 times its size? Or some giant moth flying around?? Darwin got laughed at. This just wasn't possible. It had to be something else, but what?
Darwin knew what people thought of his idea.
From Darwns book "On the various contrivances by which British and foreign orchids are fertilised by insects"
When people don't understand things (yet), some people attribute this to divine intervention. The duke of Argyll concluded that in his book 'The reign of law'. However, despite this book, more than enough botanists supported Darwins hypothesis. Hard evidence was still not found though.
Finally
Unfortunately, Darwin died before evidence was discovered: only in 1903, a moth was discovered in Madagascar, having a tongue of about 30cm. They called it Xanthopan morganii praedicta: 'preadicta' (meaning 'predicted') referred to Darwin who predicted its existance.
So: no matter how funny or ridiculous it may have sound, Darwin was right! After the discovery of the moth, it still took a long time before the pollination was captured on tape. Below you see the moth in action!
To conclude: why so tall?
What's the use of such a long tongue? Or hiding your nectar so deep? Darwins theory (and still a lot of biologists stick to this) is kind of a vicious circle: the orchid is best pollinated if the moth has to make an effort to go deep: only then, contact between moth and pollen is sufficient. So when the moth evolved a longer tongue to make things easier, the orchid reacted with deeper nectar... et cetera!
A different hypothesis is moths should have a longer tongue, because flying too close to the flower brings a risk of attack from jumping spiders.
There might be more theories; I did not do an extensice literature search on this specific subject. And disclaimerL the forementioned hypothesis are obviously more complex than my description above. Please check the references if you're interested in the scientific articles (last 2 citations).