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It wasn't that Microsoft couldn't do it, it's that they chose not to.

Through all of Windows phone 7, 8 and 8.1, the devices that were selling were low end devices -- things like the Lumia 520, especially in markets with more cost sensitivity. Because of software architecture differences, windows phone was often more responsive than android on bottom of the market phones, and that drove sales. But when it came time to release Windows Mobile 10, there were no low end phones.

Having to build out essentially three flavors of your app (one for 7, one for 8, and a 'universal' app for 10) isn't a great way to keep developers engaged either.