It's the almighty dollar.
HHN earns them a FORTUNE. Holidays are free with regular admission. There's some food, bev and merch sales, but overall, I bet HHN is a far bigger money maker.
Sea World is even worse. They only offer the full holiday event on selected days. Last year, they posted those days in late summer/early fall. I booked based upon that - then they suddenly removed those days from the calendar in late Oct - after I finalized hotels - and rates had skyrocketed in price!
I was so annoyed. Rescheduled everything - then of course, it poured the days I was there and most events were canceled.
It was NOT my year. lol.
I totally empathize. Is there anything beyond UO you'd enjoy that you could splurge on to make up for it a bit - the ice show at Gaylord, perhaps? Or a WDW holiday party?