It's a question we've already heard numerous times this morning, on various social media channels. Because while games that leverage DirectX 12's supercharged graphics technology won't hit the streets until the end of the year, DX12 itself comes with Windows 10--and a lot of people are still seeing DirectX 11 or 10 listed when they run dxdiag in Windows 10.
Back to the question: Why It's simple. DirectX 12 needs to be supported by your hardware, too. If it's not, you'll see an older version of DirectX installed.
Here's a list of all the graphics cards and integrated graphics processors that support DirectX 12:
Intel: Intel Haswell (4th gen. Core) and Broadwell (5th gen. Core) processors
AMD: Radeon HD 7000-series graphics cards, Radeon HD 8000-series graphics cards, Radeon R7- and R9-series graphics cards, and the following APUs (which meld CPU and GPU on a single chip): AMD A4/A6/A8/A10-7000 APUs (codenamed "Kaveri"), AMD A6/A8/A10 PRO-7000 APUs (codenamed "Kaveri"), AMD E1/A4/A10 Micro-6000 APUs (codenamed "Mullins"), AMD E1/E2/A4/A6/A8-6000 APUs (codenamed "Beema")
Nvidia: GeForce 600-, 700-, and 900-series graphics cards, GTX Titan series
Of particular note, Nvidia promised DirectX 12 compatibility for older graphics cards based off its Fermi GPUs--namely, the GeForce 400- and 500-series. This morning, Nvidia released a knowledge base article stating that while Fermi compatibility is still coming, it doesn't support DirectX 12 quite yet, as first reported by Anandtech. Bummer. Fortunately, the first DX12 games aren't expected until the holidays, so Nvidia has plenty of time to catch up.
Further reading: DirectX 12 FAQ: All about Windows 10's supercharged graphics tech
If your GPU supports DirectX 12 and you're still seeing an earlier version of DirectX, try heading to your graphics providers' website and downloading the latest available drivers--Intel, AMD, and Nvidia have all released Windows 10 drivers. And if you're still not seeing DX12, try rebooting your system. A couple of users have reported seeing DX11 immediately after upgrading to Windows 10, but that jumped to DX12 after a reboot.
Computers, man.