Mr. Adventure said:
Well, there is also the fact that the show took a 90 degree turn in the finale from where it was to what it is now. While it was bold and daring, that might have been a turn-off to some who didn't care for it. A possibility, maybe?
For those who were on the fence? That's certainly a possibility. But I think the number of people who weren't commited to the show by the time of 2x20 would have been negligable.