It's not really about the content - it's a McLuhanesque phenomenon of "the medium is the message". When CD-ROM became affordable for consumers, investment into content that demonstrated the power of CD flooded in.

A few years later, the investment cycle moved on towards 3D and online. Different medium, different message. That really is all that is needed to explain the trends. Galaga remains fun and playable, but nobody is marketing Galaga as the next big thing, so it isn't making sales charts.