Well, that explains it then. In the saved game I used to check for DC availability of .45, I had not completed Honest Hearts yet.
For a bit of head-canon explanation; Maybe the Capital Wastelanders used up all of their local stockpiles of .45 ammo and weapons fighting the supermutants, because the round had a pre-war reputation (from various leftover gun magazines etc) for 'stopping power' that far exceeded it's actual ballistic characteristics. It may not really have been an ideal round versus supermutants (instead it is actually better to think 'hunting kevlar-armored elephants' when choosing a proper round to shoot at the muties), however, hence the total ammo consumption. The .45 rounds would then appear in the capital wasteland only after the trade caravan networks obtain the .45 ammo/specifications from Zion.. (That is, if the Capital Wasteland vendors indeed do get the .45 stuff after Honest Hearts.)
(Sidenote: IRL .45 is not all that much different from 9mm parabellum performance-wise; .45 has a bit wider wound channel in meat, but a bit less armor penetration - the real difference is something like 10% AFAIK. However, this does not stop the '.45 Cultists' in gun enthusiast circles from all but worshipping the round. I guess this is a bit similar phenomenon as 'Katana Cultists' are amongst sword enthusiasts.)