The ISO is an option to buy a stock, not actual stock. An option on a stock has material value due to the de-risked nature of it.
Someone with 'preferred' shares gets their money back first, sure, but someone with an 'option' never even has to put their money in the first place. An 'option' gives you all of the upside with none of the downside.
Would you rather put $1M in a company for 10K 'preferred shares', or would you rather be given an option to buy 10K shares at 1 cent each. Without knowing anything else at all, you'd probably choose the later.