The problem with adding Forged maps is that a lot of them can have game-breaking bugs or unintended exploits that can be exploited in game, which just aren't very easy to detect: look at Settler, for example. This is a map which was Forged by 343 themselves, and even then had the infamous respawn death issue. The other things is that while Reach undeniably did have a very large pool of Forge maps, a lot of them simply weren't very good: 'The Cage' was so broken it had to have a major overhaul to its structure within months, 'Paradiso' was completely one-sided as soon as one team took the hill, and even 'Pinnacle', which was nothing less than a remake of a pre-existing and very popular map, suffered from major balance issues: and these are maps which were made and shipped by the Bungie team themselves!
As for a remake of 'Hemmorhage'... 'Hemmorhage' had a great deal of problems stemming from the fact that in addition to not quite being either a 'Blood Gulch' or 'Coagulation', its just didn't account for the weapon sandbox, which is probably a big part of the reason why it's not in Halo 4. Long open sightlines with hills forming most of the cover and occasional clumps of rocks worked fine when there wasn't that much elevation at the sides of the canyon and the main rifles were fairly medium-range, but introducing the DMR and raising the height of the canyon sides completely broke that. Halo 4 also features the Lightrifle, which is even more devastating at long range than the DMR. Put simply, it's not a map design that's going to work anymore. Perhaps for a 'Classic' playlist or something where there are restrictions on the rifles and with bigger hills and less height around the edges of the map, but with things being as they are it just wouldn't be fun or balanced to play.
And that's before we even come to the simply question of quality. I've played some absolutely amazing maps in Halo 4: I've also played some absolute garbage. And while it's obvious that some maps are bad or broken straight away, for others it only becomes apparent after very extensive playtesting, which takes time. How many matches need to be played on a map in order to make sure it's balanced? Five? Ten? Fifty? A hundred? Two hundred? Even with a team working around the clock, it could take two or three days' worth of time to make absolutely sure a map doesn't have any serious problems, let alone that it's of Matchmaking quality. Now imagine that there's two thousand maps to go through and you can start to see the sheer amount of raw time needed to make sure we're getting only good, working maps into Matchmaking. And there really isn't any point adding maps just to bulk out variety if those maps aren't any good.
As for the DLC-only playlist idea... it's been tried before, but they tend to have extremely low populations. People tend to be more concerned with what game mode they're playing than with what maps they're on, and fixed DLC playlists generally only offer DLC maps and random gametypes. If Halo 4 had a bigger population, there might be some merit to making certain playlists DLC-mandatory: but as is, there are so few players on at any time that we can't afford to start dividing up the numbers even further. If someone's favourite playlist suddenly demands an extra $20 of purchases and 5gb or hard disc space to play, it's just as likely that they'll put Halo 4 back in its box and start playing something else as it is they'll actually just pay up. If it were more popular, I'd love to see playlists start demanding DLC. But as it is, I think it would actually make finding a good match even harder than it is now, and that's a really bad idea.