When you’re buying shares for delivery or buying options, you have to pay the payment to the other party which cannot be settled using margin received from pledged shares and can only use cash.
Whereas, when you’re taking position in Futures (Long/Short) or Shorting Options, this are leveraged positions, you aren’t paying other party anything hence you can use the collateral margin, this Exchange / Clearing Corporation blocks as guarantee.
Siva had addressed this on different thread a while ago: