It was not like they didn't have a reason. The project.json was completely incompatible with the rest of their build chain, and changing it would be a colossal undertaking, that would mean they had to support two different build systems to remain backwards compatible.
I'm not in favor of the msbuild system at all, it's not good. But it works, it's reasonable once you learn it (like most other build systems), and since they're footing the bill and have to keep the entire system in a coherent state, they decided what they did.
The only reason we saw it, was that they've opened up their development process a lot earlier than they used to do.