Good posts from Ascend Charlie and AMDEC!
To add to the reasons why there is no rigid two-bladers:
The rotor head on a rigid/hingeless rotor will be exposed to very high forces. The MBB BO1XX rotor head is made from titanium which is expensive.
The mast will be exposed to high bending moment, an would also need to be made from expensive materials.
To be sure that the mast not suffer from mast moments exceeding the limits there might need to be a mast moment indicator, which also doesnt seem like a cheap solution.
As for the BO’s we know they cannot be flown without hydraulics due to high forces comming from the rigid rotor system to the cyclic and collective, making the hydraulics vital for the safety. It has to be two hydraulic system with a system ensuring that if one system fails it will not compomise the remaining hyd system ability to let the pilot control the helo. Not cheap either.
I would say that the main reason to build helos with two rotor blades is simplicity to keep the price and operating costs down.
The two blades solution doesnt mix well with the above requirements for a rigid system.