Why is it necessary to fly in visual conditions to certain unmanned airstrips? I have come across certain unmanned airstrips where only VFR flight is allowed not IFR? I wonder what can be the possible reasons for that?
Two reasons:

1) It takes time and money to create an instrument approach procedure, and possibly not enough people have requested that one has been created.

2) A certain amount of obstacle clearance is required, if for some reason the required safety cannot be guaranteed using instruments, the FAA can simply not create a IFR approach