They may have been busy elsewhere.
I don't buy that for a second. Soldiers in hot contact getting shot up always take priority. Normal recon and other missions automatically divert to provide CAS. You can't tell me that all of the helo gunships in the Baghdad area were at that moment providing critical CAS to troops elsewhere who were getting chewed up even worse than this convoy. If that was true, we would have lost dozens of soldiers in combat on that day, and we didn't. I think that this was a major fubar snafu. I remember the reports of that day, they had groups of armed Iraqis out on the highways west of Baghdad doing the allah dance for hours, and not a helo in sight. Why?