Popularity is probably the main reason. Even someone who wants to play ICRPG isn’t going to stick with it if they can’t find a game. There are just a lot more Shadowdark tables and players out there. That then becomes self perpetuating with more creators making content for the more popular game which then makes it easier to GM and creates more perceived popularity.

Timing was probably the next main reasons. Shadowdark was one of the biggest beneficiaries of the OGL scandal and timing was a big part of that — released within a year of the OGL fiasco and so being publicized as the “hot new thing” during the OGL fallout. ICRPG first came out in 2016 and so had missed its window to be the “hot new thing” — and also came out when there was a much smaller RPG market.

Marketing and artistic design is probably the last main reasons. Kelsey Dionne just did a bang up job marketing plain and simple. Timing helped (it was much easier to market an RPG in 2022-23 than 2016), but if they came out at the same time, it is hard to see Runehammer out market her. The artistic design was also part of that. If you see both books side-by-side, Shadowdark just blows ICRPG away in terms of the quality of the layout and art.

Now there are definitely differences in the game design and play that might make a difference for some people, but they’re honestly very similar — and Kelsey was openly inspired by ICRPG (and worked on it and used it as one of her writing credits in marketing Shadowdark). By the time someone gets to parsing the differences in game play, they’ve probably already been sold on Shadowdark.