It makes abstraction of the fact a novelist can stretch time to make you see all the details and lead you through each moment he wants you to see, including the plot details. In real time as on the movie/TV medium, you don't have that and need to focus more and make some choices.

The other thing is budget. The sequence did have a ton of entertainers in the background, those jugglers, fire eaters, acrobats were all there but stopped and left when Joffrey stopped the revelries to speak (in part this is because they then needed reaction shots from secondary actors, and if they wanted the audience to catch them, not having a ton of stuff happen around them was better).

The director and production team had to make some choices to both be able to show the scope of the wedding, with its hundreds of extras, while keeping the budget, already lavish, under control.

Their choice was first to divide the set in three distinct areas: the less important guests, the ones we saw "have fun", where seen only in the beginning as guests arrived and entered the more exclusive area behind the trees where the more high nobles and courtier were. Those were more sober and formal. Then they isolated the royal family, and cleared the area in front of them.

That let them film the crowd as they wished, with its dozens of circus/fair-like entertainers, but not have them circulate among the main actors. That gave a sense of big scope, but that way they also avoided having serious continuity issues to keep track of.

Had they brought the secondary cast (Varys, Oberyn etc.) to be much closer to the main table and had the entertainers among them, the budget would have sky rocketed.

The director was rather clever, following the Robert Altman style of filming such a large sequence of scenes as little stories, one leading into another, going to and from as the sequence progressed. This made it easier to follow everybody, to create some anticipation, and to keep track of everybody, while limiting to a maximum the big continuity issues that require to slow down massively the shooting pace and to have a lot of extra AD, props people and continuity people on set.

Simple things like having the cast eat a lot from scene to scene, or dishes change as the sequence progresses makes scenes like that cost a whole lot more money (at every take, props people need to get involved, compare the table with reference pictures, replace food that's now part eaten and such.. which takes a lot of time, for not much result on screen). Clearly the main cast was instructed not to touch the food props too much, which avoided those problems.

It's the same for entertainers.. if you have specific entertainment visible in the background instead of more random entertainers like they had, you need AD to direct these people between takes, and a lot of attention to pay not only to the actors but to continuity issues created by the extras. Every take the extras need to be put back into proper position and readied for a new take or for a change of angle, and it multiply the odds the extras do something that ruin a take when the actors were good in it. It's also much more difficult to light such sets. It's very expensive as it slows down filming massively. Not only that, but if your whole sequence is articulated around various entertainment numbers in the background, you lose much freedom to move things around when you get to editing and realize you'd be better to have one scene happen before another but.. oops... you see the bloody dancers in the background, so you can't do it. So not to have problems, you end up requiring a whole lot more prep time to set up those scenes and be sure of what you'll get, which TV teams can't really afford.

People tend to forget that despite the lavish budget of the series that many feature films would envy, that's still a far cry from the budget of Hollywood super productions that can afford to film very complex scenes with continuity issues to keep track of. On TV, just having so many actors all doing things in the background as we had at the main table is a challenge. It took them over five days to film the 18 scenes used for the wedding banquet.

They did a very good job showing the scope of the event without wasting a ton of money just to overfill the background. Isolating the main table and having most of the revelries happen in front instead of all around was a very good compromise.