WC1 may be moddable to a certain extent, but there are limits. The cutscenes between the series are hard-coded into the .exe file, just to name an example (in non-technical terms: that sucks).
From a story-telling perspective, there's also quite some limitations: there's Shotglass, there's two characters in the bar and there's the briefing and the debriefing. Compared to that, WC2 offers much more:
- cutscenes are not inside the .exe file
- a dedicated story-line view, making it possible to have multiple scenes
- cutscenes in-between missions (hell, I nearly freaked out 20 years ago when I first saw that in-flight conversation between Bluehair and the Concordia and then landing there)
- probably some other things that have evolved from being hard-coded inside the .exe