It's not limited to the last two decades in the US -- and to clarify, I said that ERs have been flooded with an increase of cases in the past two decades, not that it has only existed for that long. There are a dozen reasons that CHS is only very recently becoming understood and why known cases are skyrocketing, and why I can be 100% confident that CHS is both real and that cannabis is the direct cause. Here are just four of them.
1. Cannabinoid content is through the roof. Legalization created a marketing arms race between growers to develop hybrids which yield the highest possible THC and CBD content (the only two cannabinoids listed on the labels), and that resulted in nearly every available strain now containing more than triple[1] the average THC since the 1990s. Plants are effectively now at their maximum potential levels, and that's not even considering the advancements in oil production leading to levels as high as 99% THC, while also being extremely easy to over-consume.
2. We thought it was strictly a childhood disease for hundreds of years[2]. In the early 19th century, documented reports of cyclical vomiting were largely from children experiencing migraines and an unexplained slowing of GI motility resulting in prolonged vomiting. Any cases reported by adults were presumed to be a continuation of their childhood cyclical vomiting syndrome. This presumption lasted until the 1980s, so until the past few decades there hadn't been a motivation to consider other possible culprits.
3. We thought cannabis increased GI motility. Until the research flood gates opened over the past couple of decades, there was a presumption that cannabis "improved" the digestive process. I've only ever found research that refutes this idea, so it seems to be a Berenstain Bears situation where everyone thinks we had done the research and came to a determination, but we've actually been operating entirely on the anecdote that weed "makes you hungry." In reality, cannabis slows GI motility[3a] and the "hunger" is actually a combination of reduced psychological inhibition and a reduction of PYY hormones[3b]. We think we're hungry, but we're actually overfilling our stomachs to extremes.
4. CHS can be induced on demand. A person susceptible to CHS needs only to consume cannabis for a prolonged period and then stop suddenly, at which point a vomiting episode is almost guaranteed to start. Unfortunately, "stopping" doesn't necessarily mean quitting, as all it takes to induce a CHS episode is eating a meal without smoking afterward and then going to sleep. At some point during the night, any previously smoked cannabis will no longer have an effect, but it takes a long time (typically days) for the body to restart producing the required endocannabinoids to replace the marijuana cannabinoids that the system had previously been flooded with -- which is why CHS sufferers often report of waking up unexpectedly early preceding an episode. All of this temporarily results in the upper intestine and stomach valve becoming virtually non-functional, but because the stomach is still producing digestive fluids, it eventually becomes overfilled. The vomiting starts because the stomach is desperately pumping and convulsing in an attempt to push its contents through a valve that has effectively become a brick wall.
[1] https://wayofleaf.com/blog/average-thc-content-over-the-year...
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2886424/
[3a] https://pubmed.ncbi.nlm.nih.gov/22133305/
[3b] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6200580/