Your buying power is different for some futures & options contract because DA USA considers some positions too risky to lend out money against those products. They may have low market open interest, low total trading volume or high volatility.
You can keep track of how much of purchasing power you have for a particular futures & options contract here.

When using margin, your buying power is different for some stocks because we don't lend some or any money against those particular securities. Some examples of equities on which you can't use your full buying power include leveraged ETFS, newly-publicly-traded stocks, and stocks priced at less than $3.00 per share (these stocks are at greater risk of being delisted). Additionally, your buying power will be different for cryptocurrency and options contracts as they are both non-marginable. You can keep track of how much buying power you have for a particular stock, ETF, or options contract by looking at its trading page. The same can be done with cryptocurrencies through Robinhood Crypto.