Why are the unity base scripts in ActionScript?

I was browsing around in my files and wanted to check out the unity MonoBehaviour script. I searched for it and found it was located at:

C: Program Files (x86) Unity Editor Data PlaybackEngines flashsupport BuildTools AS3API_nophysx UnityEngine

And I was very surprised to find it to be an Flash ActionScript file (MonoBehaviour.as). And there were all the other scripts like GUI, GUILayout, etc. and they were all ActionScript files.
Now My question is, why are all the unity base scripts in the ActionScript language?

Edit:
I just realized it didn't take the backslashes in the directory description. I added spaces instead.