My two cents.....Janeway selling out in the first couple hours took him by surprise. He had to scramble to open up more orders to fill the demand, it showed that there was a strong Voyager fanbase, or at least a stronger interest in Voyager then he expected. This is why I think the Voyager crew is the first crew that will be finished. Well that and the relative ease of getting them done.
He's said time and again that each "line" has its own production lane, so rotating through one series each month was never on the table, and we've seen proof of this with the randomness of figures, and the teaser shots of various figures. However, "something" has slowed things down, be it the approval process, something in production, or something financial we just don't know and probably will never really know.