Welcome to Dhan @skslinux. I will have to check with our Risk team on this, but my guess is it may be for the liquidity reasons that State Govt bonds are not permitted.
Also margin benefits on Dhan are not just limited to Options Selling, but Options Buying as well. We had written more about this, you may find it useful: Full post: Margin Benefits & Pledge Shares Experience for Trading on Dhan