It's probably because they couldn't add the customization stuff to the UI.

That's why we got campaign but no Firefight. Your custom character is only in Firefight while Campaign models are preset. That's also why we probably won't get Reach in the MCC, not even just a Campaign mode, because they would also need to include some for of character customization which could be a lengthy process and consume a lot of time and resources.

We're more likely to see Reach as a standalone game on the market, but I doubt it'll ever be included in the MCC for those reasons.