1. Why Tatarstan was succeeded to be the “elected” region in terms of its’ authorities?
As far as I could understand, the reasons why Tatarstan was chosen for delegation of Authorities and this Agreement are mostly historical. Historical and political preconditions of Tatarstan turned in such way, that Russian Federation and Tatarstan were nearly in equal position. For example, Tatarstan was independent co-founder of USSR, Tatarstan Tatarstan did not participate in election of State Duma in December 1993, Tatarstan did not sign the Federative Agreement in 1992. This shows the level of independence of Tatarstan from Russian Federation.
In my opinion, Tatarstan was not very powerful republic, thus, to keep on developing it needed an alliance. It had political independence, but in terms of resources and power it did not have much. Russia, on the other hand, wanted to avoid appearance of another country just in the very heart of it. To avoid repeat of Chechnya Case, Russian Authorities decided to change the way they behave about it. Instead of gaining power using military forces, Russian Federation took the way of diplomacy and political alliance.
Thus, we can say that both sides needed this agreement, bith sides could benefit from it, but in my opinion, Tatarstan would need it more than Russia, in the case of different location.
2. Suggest some more or less objective criteria to select regions for establishing special favorable regime of relations with the federal center.
This question in my point of view cannot have theoretical solution, because in any different case there would be new accomplishments and new terms, that would challenge the theory pretty much. But still, some tips could be defined.
I think that in century of innovations and technological development, very important aspect would be potential rate of development of the region. It’s not important how much the region is developed,