I’ve never done a sportive, so can only talk from impressions that I get from the press.
The big audax is PBP, a big sportive would be a TdF stage etc.
Audaxers test themselves against the course and other amateurs (irrespective if the amateur is hard as nails, or soft as sponge). Sportive riders test themselves against the course, and by some abstraction the professional athlete.
The marathon analogy for a sportive does fit. Unfortunately whereas people see a sportive as a marathon, they see an audax as 50/100/200 mile fun run.
Mudguards, speed limits and lights do give us a strange reputation. From the point of view of a sportive rider, you spend hundreds to shave grams of weight off your bike, then you’re ‘forced’ to put commuter comforts onto your race weapon. A bit like having a F1 car, and then driving it with a tartan blanket over your legs, and a tin of boiled sweets on the dash.
How many people on a hilly hundred or 200 would actually bother the 30kph avg speed? I think many people mistake the average for a top speed, which would also put many people off.
Audax also has a progression, some mistakenly see the 100s and 200s as the ‘beginner’ events, compared to the 400s, 600s and 1200s.
In many ways, what attracts us to the audax, puts off many other riders. I like using my brain to read a routesheet (it distracts from the pain). Half my anecdotes are regarding off route errors, mechanicals or food at cafes.
The idea of riding really really hard for 5 or 6 hours following signs attached to lampposts, fuelled only by sports nutrition drink and energy gels really doesn’t appeal to me.