Quoted from mbeardsley:Actually, I really doubt that the cost of a larger ROM was really ever an issue. The cost of developing the extra speech/music/video would dwarf the couple of dollars per machine that a larger ROM would have been.
90's games were less "deep" because it was unnecessary, not because they couldn't do it.
Another key reason why games then were less complex: the more complex you make the code (especially with things like multiple modes running at once that can interact with each other), the harder the code is to develop, the longer it takes, and the more likely it is to require post-release patches to fix bugs.
These days, it's routine for pins not only to have post-release bug fixes, but to have post-release feature implementation. That would be almost unheard-of back in the 90s, especially the early 90s, when upgrading a pin on location required swapping EPROMs.