The problem is, instead of designing for these features from the get-go, Relic decided to stick with a nearly identical replay structure from vanilla. The replay file is formatted almost exactly the same as in vCoH, and contains extremely similar information.
For certain things like rewinding replays and observing, drastic changes would have to be made to both the replay file, how that replay file is written, and how the game engine runs replays. This would've been a lot easier to do if it were planned from the beginning.
Replay skipping wouldn't be hard to implement at a basic level. Just save the entire state of the game at a regular interval (~10 seconds) and load the nearest one to where a player skips to. Multi-player observing is also probably easier than you might think, you could essentially host a game with AI who follow the instructions given by the replay. obviously not easy or quick to do, but something that I'd be disappointed if we didn't see post-launch.