Because these are two very different industries when you look at their markets;
Solid rumours I'd put any money on come along very rarely. I personally discount almost anything that's more than 4 months out from an announcement.
I still can't fathom why the camera makers are so secretive, when companies like Intel and Microsoft provide roadmaps into future announcements.
The regular customer doesn't buy much direct from them.
Okay, then why do automobile companies do something quite similar, and telegraph their model replacements or refreshes usually more than a year in advance, and their technologies many years in advance?
They showcase models when they're done developing them, prior to general availability.
Before that, everything is hush-hush. Here in Norway (and also Sweden) each year you get spy shots of cars from the large european manufacturers being caught while making the journey to the north for winter testing etc. these cars are masked and hidden away as best as the manufacturers can. (like Canon/Nikon do when field testing their I'm sure)The manufacturer won't confirm or deny anything and their reps tend to flee at the sight of a camera.
When a model or technology is ready and introduced it is because it is meant for mass consumption, and they are sure of if and when they can deliver.
I don't really see much difference between the auto industry and the photo industry here. Models are introduced at for instance trade shows. Apart from concept and futuristic tinkering (as Canon have also done, for instance the 4k prototype..) very little is spoken about when it comes to new model lines or continuation of existing ones.
(Canon have made development announcements, like the 1Dc and the 800mm. It is because it serves them some purpose. They are limited in scope, one product typically and assure customers that they too are in a certain game.)
Because if you lay out your plans in the open you also make a commitment on delivery.
I work for a major software and hardware company in the IT industry. We do not (and are forbidden) to discuss plans of upcoming features, versions etc. yes there are road maps, but they are mostly internal, non-committing, often under NDAs. The public roadmaps are very broad, not much detail in them.
These things have financial implications; law-suits etc. from investors, customers etc. if you do not deliver. In an established marked I think it's better to keep quiet and surprise the markets every now and then than to be very outspoken and disappoint.
We had a (future) product once, the SPARC ROCK CPU which was a very unconventional new CPU design. We started talking about it in 2005. it was cancelled in 2009 because we could not perfect the technology. It cause a lot of negative press each and every time we delayed it much to the dismay of investors and the markets.
my shots on flickr https://www.flickr.com/photos/trondstromme/