343i itself admitted that the game had many flaws and issues, things that had been overlooked in Halo 4. Everyone else noticed this as well. We were sold a game only about 70% done.
After the reveal of Halo 5, IMP, it is clear that 343i obviously knew the release timelines of all the halo games, and that if they would have waited the extra 4 months it took to bring Halo 4 to a fun, reasonable game, E3 would be right around the corner, that would not look very good.
tl;dr-343i rushed Halo due to Microsoft’s release timeline of the next trilogy. With E3 around the corner, they couldn’t wait to release.