My brother used to work for a division of Chilton's Automotive mechanic shops.
Fromw hat he told me (although this was nearly ten years ago) they require the car to have a certain percentage of sales, and they also look at any aftermarket specialties for the car that are being imported, or created, as well as whether the car has a large base of followers that STICK with the automobile before they'll begin their dis-assembly procedure.
I'd assume the reasoning behind this is so that they know whether they'd make a reasonable amount of profit for the EXTREMELY labor intensive process of dis-assembling EVERY piece of that car, and notating it's placement, head size, bolt size, what it connects to, wehther it's fused, or grounded, etc.
It's a complicated procedure even for the most experienced mechanic, and it's for that reason sometimes especially with new, or Re-manufactured lines of cars it can take a couple to several years to see a manual from this company in specific.
I'm not sure about haynes' policy, or the likes, but again this is what I've been told by someone that used to do the very actions the guides require.
He also told me that there's a 25 point checklist that every removed portion/part/piece must go through noting it's placement, size, location, what it's connected to, what tool removes it etc. so that's another reason these things probably take time.
I'd also assume although on this note i'm actually unsure, that they wouldn't just disassemble ONE vehicle, as they'd have to have documentation for every sub-model, and variant of that car the factory develops, and if I remember correctly my count was that the mirage has 15 possible variants.
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View my fuel log 2014 Mirage ES 1.2 manual: 36.2 mpg (US) ... 15.4 km/L ... 6.5 L/100 km ... 43.5 mpg (Imp)