There are two main reasons:
They are averse to using the Disney brand on anything which isn't family friendly. 20th Century will be producing content which goes beyond that remit. You won't see the 20th Century brand on Disney+ either.
Although Disney is obviously a huge corporate giant they don't like to seem that way, which is why you have never seen "A Walt Disney Company" on the logos for Pixar or Lucasfilm either. It gives the impression of autonomy, and all the creative freedom which comes with that.
Last edited by Whataday on 6 February 2020 2:36pm
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