One reason I can understand is that they don't want competitors to find out the features of their product hence releasing their own product before they get a chance to release it themselves.
I believe it could also backfire like in the case of Segway which when it announced the code ginger project turned out to be just hype.
What are some other possible reasons Apple uses this strategy, keeping everything top secret until announcement day even though other companies are also releasing tablets as well?
Some companies (and Apple is one of them) can generate enough advertising and interest by saying nothing. A media frenzy begins, people blog and report rumours, people chat at the office or in a bar and all this costs virtually nothing.
Having said that, don't think that Apple are doing nothing. They will have marketeers who are carefully ngineering some of this behind the scenes to get maximum exposure.
When I was a kid in the 1980's there used to be a software company (Ultimate Play the Game) in the UK that wrote computer games. They never gave interviews, they never showed screenshots, they never advertised until a few weeks before the release and even then these adverts were just a picture of the software box and a name, but they were the most talked about games company of their time. In fact, they still are today. I don't know why, but some companies pull it off well, while others fail miserably.
You might find this article interesting: How Apple Does Controlled Leaks. In a nutshell, keeping new products secret generate even more buzz ;)
Alfred Hitchcock said, "Action is when there is a bomb under a chair and it blows up. Suspense is when there is a bomb under a seat and it DOESN'T blow up." The build up of suspense creates a drama and excitement.
Ever go see a movie after you've been watching the "dailies" for the last 9 months? Not so interesting.
Because they can.