... on a series that is over 4 decades old.
Studios make a lot of money from catalog titles, especially ones with particular value, like the original Star Trek
and its spin-offs. It's no surprise to me that they're no longer offering the series for free.*
In the very least, it (along with the other live action television programs) is also available via Netflix's streaming service, nightly in syndication, and about a dozen different home video releases.
*(And by free, I mean on a solely ad-based basis)