This is really off topic as this Xbox stuff has zero to do with Glassware, but I believe they didn't include backward compatibility for strategic reasons.
The Xbox One was spec'd out years before launch. It would have been straightforward *for Microsoft* to build it out and include it, if they wanted to.
The reason they didn't is simple - in 2013 Microsoft told the Wall Street Journal that the reason they weren't including it is that based on Microsoft surveys only 5% of customers play older games on new consoles, and only 12% of customers would be upset with no backward compatibility.
It was therefore thrown into the "not now, maybe later" bucket.
Sure there are technical challenges with a new architecture but it is relatively simple for an organization with 45,000 engineers (i.e. Microsoft).
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