Why are market orders blocked for deep ITM index options?
Market orders are blocked for deep ITM index options as they are usually illiquid contracts with low trading activity. Hence, the bid and ask spread could be very high and far from the contracts' last traded price or theoretical price. Market orders could get executed at a far away price affecting the P&L as the market order is an instruction to buy the specified quantity of a stock at any available price.
A limit order can be used as a market order to overcome this. To learn more, see How to use limit orders as market orders?