I was surprised that Lancer was that high (although a distant 3rd) when it came to "users for whom it was the most played", but on reflection, less so -- FoundryVTT being at the heavyweight end in terms of complexity and automation capabilities would make it more useful for systems which are at least moderately crunchy and have a lot to potentially automate.
For a game like, say, Delta Green, one could run that perfectly well with just Discord, sharing images assets and using voice chat and a dice bot (unless everybody has their own dice, ofc), and maybe sharing character sheets on Google Drive or w/e. It's perfectly fine to run it without a battle map and doesn't need a detailed combat tracker with automation for a dozen different conditions and effects. But if I'm running something with crunchy detailed tactical combat (say, Pathfinder 2E), having all that tooling matters. In my case, I use a different high-complexity VTT (FG), but the reasons are similar.