This is because streaming rights for shows are constantly changing. The rights to "Once Upon a Time" have likely been acquired by another streaming service or network.
Here's where you can find it now:
* Hulu: It's currently available on Hulu.
* Disney+: As part of Disney's acquisition of ABC (where "Once Upon a Time" aired), Disney+ now has the show in their catalog.
It's important to note that streaming rights can change at any time, so always check the most up-to-date information on the streaming services' websites.