The problem is that not everyone knows how to play RPGs. I take part in Encounters campaigns (among more traditional campaigns), and there is a pretty clear divide between veterans like you (multiple editions, experience DMing, good at on-the-fly roleplaying) and new players (usually only 4E, never DMed, not great roleplayers). It then pans out that there are plenty of veterans who can and do DM, while the new players are scared off by the amount of experience they think they need to have (for instance, the players know the rules better than they do, so they call out mistakes frequently).
They want to try and convince these newer players to become DMs, because that's the only way to ensure that D&D continues after the old guard retire or die off. Therefore, they are focusing most on trying to educate this new group and draw them in, while they trust that veterans like you know how to take campaigns and adventures from the back catalogue and adapt them to fit your needs. It kinda gives the shaft to the most loyal customers, but it's makes business sense.