3.) Cost - It costs more for studios to make movies today. From explosive salaries because actors are no longer signed under contract by studios, to explosive SFX prices, films that draw audiences need to spend more. And the price reflects it.
We'd need cost AND gross in order to be able to at least get a grasp of how good a movie did. Unfortunately even BoxOfficeMojo doesn't have costs for the older movies, and as Buzzkill
mentioned we're not exactly clear on how much these movies cost today