It's necessary to balance an original voice and perspective with fidelity to the voice, characters, and continuity of the work you're tying into, and authors who are used to doing their own original work can't always make that transition -- can't always adjust their own voices and sensibilities enough to produce an authentic and satisfying tie-in.
It's really interesting that you write this, Christopher. I am not sure tie-in fiction needs to do this, that in some senses authors should have the chance to do very different interpretations of characters, events and the like. (perhaps a very Abrams-esque, be gone with the sacred cows approach - and not at all commercially-minded?)
I think of it like artists approaching a particular iconography, there are some things that remain the same (the contents, say Christ, the Virgin and St John the Evangelist in most crucifxions) . However the artists' intepretations of the event and traditional elements are dissilimilar (stylistically or formally, compositionally, allusively), so much so that the basic iconography is completely remade, and each achieves very different effects for the audience.
In some senses I do wonder if that is what I like about the better written older books, like Ford, or a singular entity like TNS
- they are so much more distinctive reintepretations of the base.
If one had Zadie Smith, Ian MacEwan, Sebastian Faulks and Iain Banks write Treklit, wouldn't it be a total shame if they were not given the chance to write their own version? I guess the question that underlines what I write, is what is 'tie-in' literature and what can it be and not be.