[I may or may not hold the following views.]
1. Tobi was partially an intriguing character due to his hidden identity.
2. Obito was an obvious candidate and theorists believed Masashi would not choose such an obvious path.
3. Obito's overall reasoning is weaker than Madara or Nagato.
4. Tobi 'acted' differently; Obito appears to be less patient and more emotional than Tobi.
5. Theorists were upset that their theories were proven false as some needed a lot of time to compose.
6. Out of all the candidates, Obito seemed most prone to the 'talk no jutsu' notion.
Obito being Tobi was only made obvious due to the structure of the war. Imagine if Itachi and Sasuke later joined the battle against Tobi and Tobi was Shisui. Shisui, being the origin of Itachi's ideology is similar to Obito being the origin of Kakashi's ideology. Shisui can also be a foil to Naruto if it is learned that Shisui and Naruto had similar personalities. Moreover, Shisui would also be important to Sasuke due to their strange relationship. Kakashi, Gai and Kirabi know of and/or might have some sort of relation with Shisui as well.
In the same situation with Sasuke and Itachi joining the battle against Tobi, Fugaku Uchiha and Future Sasuke were potential candidates as well. Future Sasuke would appeal to all characters due to Sasuke playing a central role in the tale. Fugaku would appeal to Sasuke and Itachi; Kakashi, Gai and Kirabi may have a relation with Fugaku as well. Moreover, it will further hint and and emphasis the Sasuke-Naruto relationship. Tobi [Fugaku] fought Minato -- Fugaku being Sasuke's father and Minato being Sasuke's father. Moreover, Fugaku being Tobi could have been seen as obvious as well due to Tobi and Darth Vader of Star Wars having stark similiarities.
Izuna would not be significant to any of the characters other than possibly Sasuke but he certainly could be; not much is known of Izuna and even less at the time; Izuna may have some special relationship with one or more of the characters. I remember some theorizing Izuna being Sasuke's ancestor. Kagami, I can't imagine would be significant either. However, Tobi's identity didn't
have to be significant. Knowing Masashi however...
People had disputes over Obito being Tobi due to the only true evidence being his utilization of a technique similar to Kakashi's Kamui, spiky-black hair and possessing one eye. Tobi using Izanagi against Konan showed his possession of two Sharingan while Obito was only "supposed" to have one. In addition to the said, Tobi stated he "gave" Nagato the Rin'negan. We know now that Obito said this as he was acting as if he himself was Madara; at the time this knowledge was released, it was interpreted literally. Minato fighting Tobi but not realizing it was Obito was used as an argument against Tobi being Obito.
Overall, Obito being Tobi was too obvious and not obvious enough at the same time which led to some disapointment and resentment of the figure that is Obito. Also, Tobi was cool.