As the owner/founder/author of a bootstrapped SaaS app, I can answer that:
People really, really, REALLY do not want to pay. Really. The dominating perception is that software should be free, or if not free, paid with a one-time purchase in the single-digit dollars. The aversion to paying is huge, you have to provide a "can't live without" feature for people to (grudgingly) pull out their wallets.
Also, if you actually do the numbers as a business owner, it turns out that consumer SaaS apps are difficult to sustain financially. With a small number of customers, there just isn't enough money, and if your customer base grows, complexity and costs step in. My SaaS only has paid business tiers for now, starting at $39/month, because much as I try, I can't see how I can make anything cheaper work. It's easy to underestimate costs if you've never ran a business.
I decided to stick with it, charge subscription fees, grow very slowly and see where it takes me, but that is a hard and slow path. I estimate it will take me another year (for an overall of 3 years) to get to a sustainable income stream.