The only thing I can think of is it's for corporate or educational buyers who standardized on it for some reason. The 30 pin connector perhaps?
I can't find any corroboration for your assertion about the original iPad. Every source I can find says it was discontinued immediately in favour of the iPad 2. Selling off old stock to the education market isn't the same.
The main point is that it's not unreasonable to think Apple would keep products around for enterprise/education markets without selling them publicly, or at the very least without mentioning them during a high profile keynote.
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The 30 pin connector comment was in reference to this.