The old 6-day ticket was only $5 more than the 5-day ticket ($271 vs. $266). Right off the bat the daily revenue is peanuts on the 6th day compared to the prior five days ($5 vs. $53.20)
Now consider all the spending you do on a trip. Merchandise spending will probably go up very little on that additional day as most budget a certain amount for pins, gifts and souvenirs for an entire trip and is not dependent on the number of days. Food spending will probably be average or below as big ticket meals will most likely happen at some point during the trip regardless of that extra day. On top of this, you still need to overcome that almost $50 difference in revenue on the ticket.
In 2012 we had 6-day tickets and actually added two more at the ticket booth for an additional $5 per person ($2.50 per day). It was then that I started thinking about the math of these extra days and how each day just shrunk the daily average, thus making it easier to justify the total expense of our trip. On our 7th day (we didn't use our 8th) we had a few snack items in the park but ate offsite and were staying at HoJos. Our family of four rode a ton of rides, used various facilities, used our PP+, and basically took up space for about $25 total that day.
If they can keep the on site hotels at or near capacity, they stand to make more money elsewhere on more frequent guest turnover than letting people linger around stretching their budget.