There are two obvious reasons why why option volume is less than equity volume, First, a standard option is for 100 shares. As an example, an investor seeking to replicate a stock position with lower risk would only buy one high delta LT call to have an almost equivalent position. Second, options are more complex and far fewer investors/traders use them, compared to equities.
The limiting factors are the number of willing traders, their size as well as the price that they are willing to trade at.
To make things a bit more complicated, puts and calls of the same series are connected in value by arbitrage strategies called conversions and reversals. So if there's a call buyer willing to overpay, then a floor trader or market maker can take the other side and buy the stock as well as the put to lock in a gain (conversion). I say overpay because for example, it might be an option with a wide spread and the buyer doesn't realize that fair value and an executable price might be closer to the B/A midpoint. So in this case, the call buyer precipitated other buying and selling. IOW, it can also be interconnected.