You've just driven home for me why I get so irritated by statements like "You'd have a point if it were just me, but there's lot of us", implying that WotC better get it's act together and satisfy the complainers or they're toast.
It's
exactly the tone people take when the gripe about it taking so long between Song of Fire and Ice novels.
George R. R. Martin is not your bitch, people, and neither is Wizards of the Coast. Their continued success as a company is not dependent on anyone buying D&D books. If you bought the core books, you've already given them your money, and now they're looking elsewhere for customers because they'll make more money for less investment reaping profits from board games, video games, movies and the like.
Expressing you'd like to see more product offerings is fine!
I'd like to see more product offerings! But to imply they're mad idiot fools for not catering to you specifically when they are perfectly capable of deciding the best way for their company to make a profit on their own is the height of entitled arrogance.
(Not saying you're implying that, neobolts! I just keep seeing it infect thread after thread on EnWorld.)
On your other points we agree, though I think the answer as to "why" is simply that they have limited staff and don't want to announce things too far out in case they need to cancel or significantly delay a product. So far we've been given product announcements ~6 months in advance and I see no reason to not expect them to continue that trend moving forward.