Orphaned Series/Western Animation

Ralph Bakshi's 1978 animated version of The Lord of the Rings covered the first two-thirds of Tolkien's trilogy. Those who desire to see the ending, however, will be forced to pick up the books. (Which is probably for the better . . .)
Perfect Hair Forever was scheduled to have 17 episodes made for the internet but the creators only made 1 due to lack of interest.
The Pirates of Dark Water eventually simply stopped, after about eight of the thirteen treasures of rule were collected. A lack of budget and Channel Hops were the responsible issues, here.
- The Genesis game does contain a proper ending to the series however.
G3.5 of My Little Pony should have lasted at least three years, but ended prematurely at only one year. It had one special and a few toys - however, it was made around the same time as My Little Pony: Friendship Is Magic (which ended up being the series to revive the brand).
Roughnecks: Starship Troopers Chronicles ended abruptly before finishing the final story arc.
Invader Zim was mercifully cancelled when Jhonen Vasquez lost the desire to work on it.