My theory revolves around perceived power before and after a 6th edition codex. Namely, that the successful codices in 6th, particularly Tau, Eldar, and Space Marines, were generally received well because before the codex, they were not a competitive option in 6th Edition.
For Tau and Eldar, I don't exactly have a lot of proof here, and must instead rely on anecdotal evidence. Those two had very old, outdated books, and while they had some decent builds, were not making an impact in the tournament scene.
However, with Space Marines, I do have some numbers to back myself up. At NOVA, the last major Tournament before the 6th edition codex, Space Marines represented only 2.05% of the field. This small segment accounted for 1.92% of the total wins, and had a 50% win percentage. The first big event after the codex release was Feast Of Blades, where at the Invitational, Space Marines jumped to the fourth most represented army, though they still only were 5.48% of the field. However, their win percentage was above .500: 58.1%. Participation numbers increased at the 11th Company GT, where Space Marines were the most popular army, representing 17.1% of the field. However, their win percentage dipped back below .500.
What these numbers help illustrate is that, while Space Marines were generally seen as underwhelming and unable to match up to Tau, their new codex still propelled the army from obscurity in the competitive settings to one of the more popular armies.
Now, Tyranids have not been represented in huge numbers (3.07% - NOVA, 4.59% - Feast, 7.09% - 11th Co), but they were still being spared from the complete obscurity. There was always the feeling that Tyranids were still competitive, but just below the desired power level. This is the big difference I think - the perceived power level before the codex.
Because the feeling was that Tyranids were close with the existing "competitive" builds, I believe there is some fear that the Tyranids will lose ground because we have to start from scratch to develop a competitive build. Eldar, Tau, and Space Marine players all had to develop new competitive builds, but the difference was there weren't as many preconceived notions about what should be good going in.
I think, what it boils down to, is that I've accepted that the build I'm running this weekend at the SCARAB GT in a last go with the 5th Edition Codex will not be the core of my 6th Edition list. What had been the bread and butter of the Tyranid codex (Tervigons, Hive Guard) will not continue to be. As Tyranid players, we will need to discover what the new core is. I am confident that this new core will be competitive, even if it doesn't unseat Eldar and Tau as the kings of 6th Edition.