Few reasons...
1, they're much more complex to engineer and build than a standard speaker - Everything is doubled and the concentric nature makes design and assembly trickier. This makes them more expensive.
2, They don't necessarily offer any benefits that make the expense worth it. Vertical dispersion isn't super important compared to horizontal, and horizontal can be made adequately good without the difficulty of coaxial design.
3, They require quite a bit of care in designing to not have the midrange/midwoofer throw off the frequency response of the tweeter.
1, they're much more complex to engineer and build than a standard speaker - Everything is doubled and the concentric nature makes design and assembly trickier. This makes them more expensive.
2, They don't necessarily offer any benefits that make the expense worth it. Vertical dispersion isn't super important compared to horizontal, and horizontal can be made adequately good without the difficulty of coaxial design.
3, They require quite a bit of care in designing to not have the midrange/midwoofer throw off the frequency response of the tweeter.