Storytelling 101 is to tell a concise, consistent and self contained story within the medium you choose. Video games are the primary medium for Halo. People shouldn't have to read books, novels, comics, watch movies, trailers to find out things to understand the game better.
Why is this so hard for you to understand? Yes reading expanded universe canon or lore helps explain things. If people wanted to, they can wiki everything about characters and places, the problem is, they shouldn't have to. If people beat the game and are left confused about certain characters, certain plot points than you haven't done a very good job.
I didn't need to read the books to find out who Arbiter is in Halo 2. They explained it perfectly who he was, what he meant to the Covenant and what his motivation was. Can you say the same about Didact in Halo 4? Your own personal feelings aside, this was an issue a lot of people had with the game. It was evident in several reviews as well. Either way, you're taking it personally for some reason, it's great you loved Halo 4, and that you love reading the expanded universe lore.