From the article,

terrain Boring will have to navigate: the tricky, sinkhole-prone limestone bedrock of middle Tennessee. The construction risks range from collapsing the ground beneath a heavily traveled state highway, to knocking out utility connections, to flooding the tunnel with groundwater.

The ability to cut the rock is not the only challenge in boring tunnels. Regardless of the type of rocks it runs through, making tunnels using a TBM is one of the slowest and most expensive ways of making a tunnel. Its best to use other techniques, unless a TBM is the only option, which it isn't for this project.

The proposed airport line runs directly beneath sr41, and the second line is under sr70s. Just like Vegas it goes directly under preexisting roads, so they don't have to deal with the administrative headache or costs of acquiring the rights to dig under private property. In cases like this it is far cheaper to use cut and cover.

In cut and cover, you build a shallow tunnel by digging a trench, putting the tunnel in the trench, then burying it. Its the most cost effective way to make urban metro tunnels in most cases, but it does require shutting down part of the road to construct. However those carbrained enough to think chauffeur driven cars are mass transit, will consider temporarily closing a few lanes on the surface during construction to be unacceptable.