What is aerodynamic reason why turbine with 8 blades will rotate slower then with 2 blades?
It won't, all else being equal.
The speed of the turbine is governed by the aerodynamic forces on the blades and the resistance in the turbine. (By resistance I mean non-aerodynamic forces like drag in the bearings and the actual generator.) In a two-bladed turbine (or any other) the normal speed is where the total of the aerodynamic forces and the resistance balance.
If we quadruple the number of blades, making each of the added blades identical with the originals, then according to naive, oversimplified, aerodynamics we are quadrupling the aerodynamic forces on the turbine (the ones driving it). The resistance is not quadrupling, so the turbine will spin faster.
It won't spin 4 times as fast for two reasons. Firstly, as the turbine speed increases the non-aerodynamic resistance increases, and they will balance at significantly less than four times the speed. Secondly the aerodynamic behaviour changes as the speed increases, including both an increase in the aerodynamic drag of each blade and aerodynamic interference between the blades, reducing their "lift". (Aerodynamic interference is why biplanes don't have twice the lift of monoplanes. It can actually reduce the many-bladed turbine's aerodynamic force below that of the few-bladed one.) That's what I meant above by saying that we are quadrupling the aerodynamic force only for naive, oversimplified aerodynamics. These two factors means the 8 bladed turbine will top out at well less than 4 times the speed of the two bladed.
It's also important to note that 8-bladed fans aren't designed by simply taking a two-bladed fan and adding six more identical blades. The blades will be designed differently. Turbines have an intended operating speed and the blades will be designed so the turbine operates at that speed for expected airflows. Higher turbine speed does not always mean more power - a multi-bladed turbine may be designed to run at a lower speed and generate the same power as a differently-designed two-bladed one.