Everyone has to pay for marketing, and I'll give you the part about growth.
But is this because something inherent in a nonprofit stops it from growing? Is it that there are fewer nonprofits to sample wins from? Or is that venture capital is simply not structured to consider the upside of funding anything other than a bet on a huge exit?
Thinking a bit more, big software related non-profits appear to be spin-offs from industry (Eclipse Foundation, Linux Foundation) or academia (Apache Foundation also from NCSA). In each case, the software came first as was the case with Mozilla and the funding followed because corporations found they had an interest in the software's continued development.
On the other hand, OpenAI might be a counter example where the money is coming to support research so that's a possible case where YC has funded a 'non-profit' at scale...though the model was recruitment rather than application.
The Mozilla Foundation has a wholly owned for commercial entity, the Mozilla Corporation.
[1] If there was a nascent non-profit in the chain that led to Mozilla, it would be NCSA [https://en.wikipedia.org/wiki/National_Center_for_Supercompu...] where Andreesen was developing Mosaic.