Oftentimes the VC cash is needed to actually cross the chasm. One of the difference between early and mainstream adopter populations is that a lot more product polish, and oftentimes explicit sales or advertising outreach, is needed to please the mainstream audience. That all costs money.

I wouldn't say that only pre-chasm matters, though. VCs usually refuse to invest in startups that have zero chance of crossing the chasm. The prototypical example of this is say an app for D&D players. You could have the most passionate userbase, and the app could be critical to their experience, but your total addressable market maxes out at the 13.7M D&D players worldwide and it's unlikely they'd spend more than say $10 each on it, which makes it unattractive to all but the smallest VC firms.