Once again, loadouts really need to be specific to a map and a gametype. I don't understand what's so difficult to implement about this.
Increases the amount of test surfaces exponentially, leading to more things that could go wrong. It's never been done (even Halo 2 only had CTF and CTF Fast for smaller maps) because of that, and thee's also the element of player confusion, just like we're getting with the pistol. Why can't I sprint.. oh, it's Beaver Creek? Does Countdown have Hologram, or is that Reflection? etc etc.
And each time you'd want to add a bugfix or update to a gametype, you'd have to reimplement that fix x N different gametype and map combinations you have.
It's the reason that they had some map exploits last a bit longer than they usually did. There were reported exploits for Sword Base at one point, but they were in the middle of overhauling it's weapon and spawns layout. Instead of stopping the entire process and backporting a fix to the old Sword Base variant, they added the fix to version 5 and just let it ride along with that test/cert cycle for the new variant.
Same with the infamous Valhalla fusion coil. Bungie (for good reasons) only updated the playlists once a month unless it was a dire emergency to maximize time and energy spent on new variants. Since Valhalla had other tweaks done at the same time and the bug was discovered just past the update deployment, the fix for it simply had to wait for the next update deployment, no matter how simple it was, since deploying a mid-month update was reserved for dire situations (you're making the entire playerbase re-download the playlist configuration files..)
Reach DOES have map variance though for certain gametypes, flags have different return times depending on the map in CTF.
It should be noted that it was posted by Jeremiah during Halo 2's run and Shishka during Halo 3's run that both of those games only supported a finite number of map/gametype combinations in matchmaking at once. It's never been mentioned anywhere if Reach has a limitation like that. So there's also that to consider.