Two reasons:
(1) It takes a lot of iteration to get good at writing adventures for a given system. You need experience with the system, and experience with writing, (and experience with writing adventures which is a subset). While the organization as a whole may ascend the learning curve over time, each individual author is only one micro-component of the organization, so they ascend the curve very... slowly....
This means that from the time the game system launches, to the time you get good adventures for it, may be years.
On the plus side the good adventures tend to be very good indeed, as they drawn on everyone's learnings over the years. The LFR years 4, 5, and 6 Epic-tier adventures are fantastic, for example. (That's roughly "years since launch of 4e as a system" for a sense of how long it takes.)
(2) Many people are horrible in general (lazy, stupid, etc.) but as a convention organization you cannot turn down volunteer DMs. Thus, like Sturgeon's Law for D&D, 90% of all DMs are crap.
= = =
Now that said, I had 6 mostly great years of 4e Living Forgotten Realms (the RPGA / organized play campaign). Basically, my non-lazy-and-stupid-people friends and I took over the FLGS and locked out the horrible DMs, establishing ourselves as a benevolent dictatorship. Like everything else in life, you get out of it what you put into it.