From the Hollywood Reporter:
Constantin had originally planned to turn Clare's fantasy series into a feature film franchise but shelved that idea after the first Mortal Instruments film, starring Lily Collins and Jamie Campbell Bower, tanked at the box office, earning just $31 million domestically. Constantin, however, insisted it had not abandoned the franchise and planned to return to Mortal Instruments.
“It actually makes sense to do (the novels) as a TV series,” Constantin film and TV head Martin Moszkowicz told THR. “There was so much from the book that we had to leave out of the Mortal Instruments film. In the series we'll be able to go deeper and explore this world in greater detail and depth.”
Obviously this is early days yet, but I think it'll work waaaay better as a tv show.
More at Variety.