They were chasing many rabbits at once. Someone, maybe on Westeros - I don't recall, pointed out they put less effort in the Sansa red herring than they did for the Cersei one, adding that if he had only the show to go by, he would probably conclude that by far the most likely suspect was Cirsei, for whom they established her fear of her loss of relevance, her fear to be driven away, her hatred of Margaery and her spite that she was taking her place as main influence over Joffrey, her growing fear of Joffrey and her loss of control over him, that she had the means to pull it off and the cold calculating madness to hatch such a plan, her hatred of Tyrion whom she could frame for it at the same time etc. Then there was Qyburn, and her instance to send Pycelle to the kitchens to have the leftover thrown to the dogs (not sure why people believe the presence of poison in the leftovers would have been better covered up by mass deaths in the royal kennels rather than mass deaths among the poor, and they underestimate Cirsei badly when they think she also did "to spite" Margaery rather than for the much more cunning motive of angering the poor and undermining her efforts to make herself infinitely more popular than Cersei among the populace, but oh well...)

With Oberyn they missed an opportunity. They've not yet well established why he's called the Red Viper. They worked on his motives to harm the Lannisters, but not very convincingly so since they've almost established that in his idea of vendetta/justice he wouldn't punish the children or grand-children for the sins of their father. I've seen some non-reader theories that thought Oberyn would kill Tywin during the wedding. (that doesn't however beat the theory that Joffrey was assassinated by a killer hired in Volantis by Talisa's family. That's what you get for insisting not to read the books or not even wanting be told which characters are added into the show and thus "irrelevant" ).

That said, people have forgotten some not very obvious clues planted long ago, tying Littlefinger to the Tyrell wedding, and then to plotting for Sansa to leave KL with him when he left the city, and LF isn't the type to give up or take no for a final answer. They left no red flag for anyone to connect any dots in his case, though. Dontos' explanations were most obtuse, and didn't include any hint to a potential escape nor tied the necklace to the wedding the way the hairnet was (more, it was tied to revenge for Ned's death...). They sacrificed all this to preserve completely the surprise that something big could happen during the wedding itself, while Martin had used another kind of long term suspense/anticipation altogether in the book, which wouldn't have worked very well on TV.

I know from reading an interview they also worked to plant red herrings to mislead the audience that Sansa/Tyrion or Margaery/Loras were the ones actually in danger at the wedding, not Joffrey. That's why they chose to have the Jaime's threats to Loras take place at the wedding itself, notably. So I don't know if some of the "clues" pointing to Cirsei were not there to make it appear she had plans for Margaery, instead of making her suspect in Joffrey's death. For sure I doubt Lena's acting in the last scene, which some take as another clue, was intentionally "not enough grieving" or that she mean her grimaces to be mistaken for sly smiles...

I don't know why they finally left out Tyrion figuring out that Joffrey was behind the attempt to kill Bran.

My theory is that for one reason or another these lines and maybe a brief exchange afterward between Sansa and Tyrion might have ended up in the bin during editing, or were excised from the script at some point and not even filmed. Perhaps because it didn't work well with the rest, or ended up being too confusing, or they decided no one would care enough anymore to distract attention with that now.

What makes me think that is that Martin's script included a clear reference to the fact Bran was pushed down by Jaime for having seen him and Cersei. That seems to have been meant as a reminder of what happened, the sort of thing you'd expect to be included if they really needed that reminder before referencing again those events at the breakfast/wedding. Those elements in the vision were a bit pointless now.

They might also cheat and have Littlefinger know the truth about it and tell it to Sansa next week if she shows any sign of not being happy enough of Joffrey's death.... "It was also him who tried to have your brother Bran killed you know".