There are 2 main reasons for that:   First, back in 79-83, cartridge ROM chips were very expensive, so most games were 8 or 16kb in size. Today this is no longer an issue and the games can be 100 or 200kb big, so with all that extra space we can add more variety and make them more similar to games from 1986.   Second, today we use coding techniques that are way more advanced than those back in 79-83, simply because noone invented them yet.
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