Because bronchiolitis obliterans was never attributed to smoking before, and that diacetyl hasn't been actually studied as the cause of bronchiolitis obliterans before it the "popcorn lung" ordeal.

The warning against diacetyl was issued by OSHA after several cases of bronchiolitis obliterans (and if we are completely frank then lawsuits which lead to some investigation) were attributed to working with artificial butter flavorings for microwave / easymake popcorn.

Now the amount of cases was and still is negligible and while bronchiolitis obliterans on it self is extremely rare cases which can be tied even circumstantially to diacetyl are unicorns.

Here is the entry for Wikipedia regarding the causes:
Bronchiolitis obliterans has many possible causes, including: collagen vascular disease, transplant rejection in organ transplant patients, viral infection (respiratory syncytial virus, adenovirus, HIV, cytomegalovirus), Stevens-Johnson Syndrome, Pneumocystis pneumonia, drug reaction, aspiration and complications of prematurity (bronchopulmonary dysplasia), and exposure to toxic fumes, including: diacetyl, sulfur dioxide, nitrogen dioxide, ammonia, chlorine, thionyl chloride, methyl isocyanate, hydrogen fluoride, hydrogen bromide, hydrogen chloride, hydrogen sulfide, phosgene, polyamide-amine dyes, mustard gas and ozone. It can also be present in patients with rheumatoid arthritis.[9] Certain orally administrated emergency medications, such as activated charcoal, have been known to cause it when aspirated. Additionally, the disorder may be idiopathic (without known cause).

If you look at the amount of causes listed and the last sentence that states that the disorder may as well turn out to be idiopathic building a scare against E-Cig's is silly, no one knew about bronchiolitis obliterans until when in 2007 Wayne Watson who used to eat like 10 bags of popcorn a day sued and got 7M in damages and the media have picked the popcorn lung story and ran with it as well as uncovering a handful of cases dating from the early 2000's where OSHA changed the regulation regarding diacetyl requiring respirators and ventilation during processing.

Now don't get me wrong diacetyl could cause or contribute to bronchiolitis obliterans or conditions that would allow the disorder to develop, but it could also be harmless or completely unrelated to the disorder, and even if is a cause so far all data we have points to the fact that it's extremely rare factor if one at all.
This is what isn't being reported a proper report should read: "A study finds that diacetyl, a compound that might be linked to bronchiolitis obliterans is present in flavored E-Cig liquids
Bronchiolitis obliterans also known as popcorn lung has been attributed to exposure to diacetyl during manufacturing of butter flavored microwave popcorn in a handful of cases in the United States.
Diacetyl is also present in normal cigarettes in levels 50-100 times higher than the ones observed in E-Liquid. Smoking was never suspected as to have a direct to causal link to bronchiolitis obliterans as such while we advise some caution to be taken, there is no clear risk of bronchiolitis obliterans due to exposure to E-Cig vapor."

But that report will not really catch any head lines so: "E-Cig's vapor will give you one of the rarest lung diseases" is used instead.