"...Why wouldn't companies let consumers know about their latest technology ASAP and steal some sales from their competitors? ..."
Well, not only would people stop buying their competitors' products, they would stop buying their own as well...
Nobody likes to get stuck with old technology - and for that reason, products that are in the pipeline are kept under wraps to a certain extent.
If you knew of every product currently in development - what reason would you have to buy what's on the shelves today?
🙂