seems bizarre they didn't do that from the off
It isn't that they didn't want to do it, it's just that the new campaign format presented technical challenges they needed to solve in order to do it right. They could always easily make you go back to previous missions if they wanted to quite easily. I don't believe that was ever the issue.
Want to know what the technical challenge was? Making you back track and having the game remember all the new data points that needed to be kept track of and maintained, data points that previously never needed to be tracked in previous Halo games.
1 - Killed a high value target and collected their weapon for retrieval at a FOB? That enemy needed to stay dead, their weapon needed to always be in your collection at FOBs.
2 - Rescued a marine squad by answering their distress call? That needed to be shown as "complete" still.
3 - Captured an FOB? That needed to stay captured.
4 - Defeated a banished outpost? That needed to stay captured.
5 - Collected a certain number of Spartan Cores? You needed to maintain that number.
6 - Performed a certain number of Armor Upgrades to your Chief? Those upgrades needed to be maintained.
7 - What if you completed only specific objectives at an outpost, but not all of them, running before you had a chance to complete the rest? You need to return with the current remaining objectives being exactly the same. It couldn't just start over on you. (Come to think of it, I've never tried this....)
The basic things they didn't have in place had to do with fundamental changes to the campaign into a more open world structure. Imagine the hell they would get if they had campaign mission replays that resulted in people losing what they collected? Losing valor, which is the currency built up from completing tasks around Zeta Halo, giving you access to better marines, weapons, vehicles at forward operating bases. So better to just leave it out till you get it right.
And we should be glad they took their time on that one because the solution they arrived at is pretty damn impressive. It's basically like quick resume multiplied, but totally in game. The game for the upcoming november update consistently keeps track of two states for the player at all time. A replay mission state and a present game state. You can quickly bounce between both and pick up EXACTLY where you left off, in the middle of the same fight with the same enemy, in the middle of still falling from the top of a mountain etc. They also implemented multiple save states, so people are free to separate their playthroughs. They all show this on youtube.
So they took their time implementing it, but they made it DAMN good. Watch till 31:34
Wherever you are in your current campaign objective, it will resume exactly where you were objective wise, like after defeating that brute boss at the excavation site. And the same works for whatever mission you currently have set for campaign replay and look at the interface things they did on the tacmap to make it clear which ones are currently activated. These may seem like small things, but this is impressive stuff and it shows they took care to get it right.
Early on when the game first launched there was an issue where armor lockers you would collect that gave you armor for MP, wouldn't actually show up as being owned by you in the MP. I think the issue was tied to quick resume.