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?
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
I’d have to examine the specific airport, but you are required to have an approved instrument approach procedure, with current weather minimums above the published minimums, in order to approach and land at an airport when the weather is below VFR minimums for the airspace the airport is in.
Some airports also have terrain and other nearby obstacles which may make it hazardous to takeoff and land from in marginal weather or at night. Check the Chart Supplement for that region for further details.
This does not prohibit flying out to that airport under IFR, though. But the local weather at the airport must facilitate the ability to descend from the MEA and make an approach and landing under basic VFR. Ref §91.169 (c)(2)