Two completely different operating systems. It's not strange, if true, that the Xbox One can't handle two different install discs for one executable. Why build an option when you don't expect it to be used in the near future, you'll dedicate your valuable dev time for other options that are more likely to be used in the near future.
Microsoft, and I'm guessing a lot of us as well, didn't expect games to require more than 50GB of storage this soon in the generation.
If it were solely a multi-disc issue, the content update would have been part of the original download. Don't forget that there's also a delay for Spartan Ops.
Development time is probably the bigger factor.