Not forgotten! Definately NOT forgotten- just very, very foreign to the USA. This is for 2 reasons-
1) When heroin (as well as pretty much all the morphine salts listed below) were discovered europe was in the grip of a TB epidemic. As the treatment for TB was extremely crude back then (as well as not knowing much about the condition itself) many European chemists focused their intentions on creating powerful anti-tussives (things that supress your desire/need to cough). That is why, say, nicomorphine was discovered in Europe and has only ever been prescribed in Europe. They were not really trying to create analgesics, let alone anesthetics, they were trying to develop anti-tussives in a misguided belief that they assisted in the treatment of TB.
The USA, with a far lower population density as well as a slower rate of industrialisation did not face the same problems with TB that Europe did in the late 19th century- so they didn't need a thousand different powerful opiates that also happened to be strong anti-tussives. This is also why DXM was developed- attempting to find a strong anti-tussive that exerted no analgesic effects (pity about all the NMDAr anatagonism

), look at the structure of DXM/DXO and compare them to teh structure of codeine/morphine. Interesting stuff- removing/rearranging a few atoms/bonds can completely change a chemical while retaining the same basic skeleton.
2) Until after WWII the USA was largely reliant on opium produced in Turkey. As Turkey shared a boarder with the USSR US-Turkish were often very delicate (part of the arrangement to remove the nukes from cuba was for the US to remove its nukes from Turkey- you can see how all sides were tense) and due to the US inability to be self-sufficent in opium production (I have no idea why they didn't just start growing their own poppies) the decision was made to develop entirely synthetic opioids. This was initially based on the fact that theibaine (which is a precursor to many semi-synthetic opiates) was cheaper to produce than morphine. I suspect the fact that the US government seized/voided all German patents registered 1933-45 (thus meaning that the US govt got the patent for methadone and oxycodone as 'reperations', for instance), as well as giving safe-haven/protection from prosecution of many of Germany's best chemists played a big part in their thinking. And that's why when I got my wisdom teeth pulled I got some NSAIDs and a big cup of harden the fuck up, while Americans often get percocet/vicodin when they get teeth pulled.
The situtation changed after the French (who love giving the US a big 'FUCK YOU! HAWHAWHAW') bred a variety of opium poppy, 70% of the alkaloids which it produces being theibaine (it produces virtually no morphine though). This is a brillant example of the potential for biosynthesis- if you've read tikhal you'll know how excited biosynthesis made Shulgin. Which leads us to the modern period, where the cold war has ended and 80% of the worlds poppies are grown in tasmania- both for Theibaine and for Morphine (the morphine, ironically, is mainly used to make codeine- I believe most pharm morphine is actually synthetic). I believe that Afghanistan should be rebulit around the opium poppy- with the growth of the Indian and Chinese chemical synthesis industry we may actually see this happen. Farmers should be given specially bred pharmacutical seeds and they should be paid much more than the heroin manufacturers could or would. Boom- Afghanistans thriving economy is actually legal, legitimate and making the world a better place. If every Afghan opium farmer formed a cooperative that negotiated as a group with pharma companies (obviously initially with help from the UN, the Red Crescent and various Islamic charities) then they would end the war in a month and the Taliban would quickly run out of money to fund their operations in Afghanistan (being based in Pakistan and all).
There is 'The Emperor Wears No Clothes' style book that needs to be written about the maddness that the US pain management system and its geopolitical effects. The world would be a very different place if Britain forced the world to follow their pain-management system which incorperates diamorphine (the polite name for heroin). People are all up in arms about weed- which they should be, but prohibition as a whole is maddness and fentanyl/oxycodone/hydromorphone is yet another example of the maddness that prohibition has created- 'Here's a prescription for some completely pure, uncut mu full agonist that is stronger than that evil dirty drug heroin'. Then it costs more than heroin once you get it filled- awesome!
I have taken nicomorphine (which is virtually indistinguishable from heroin), dihydromorphine (which I reckon is one of the best opiates in the world) and diacetyldihyrdomorphine (better than heroin- the rush has legs, it lasts a good couple of hours longer and is equally potent as heroin imo).