The lack of belief and investment is not the root cause but a symptom - you had a limited budget of motivation and belief, and you ran out of it before getting a MVP. If you had "spent" that very limited motivation-time-runway more sparingly on a more-minimal-MVP, perhaps it would have been sufficient.
I.e. I'd suggest looking at the constraints the entirely opposite way around than "weren't ready to invest the required time/energy" - in this equation, you can (and must) change the "requirements" for time/energy, as the time/energy available for the project is your core limitation that you should treat (in the short run) as out of your control, especially as it concerns the time/energy of others before funding. From the classic project management tradeoffs, this is a "limited budget - adjust scope to match" scenario, not "fixed scope - let's see what the budget will be" one.
I.e. I'd suggest looking at the constraints the entirely opposite way around than "weren't ready to invest the required time/energy" - in this equation, you can (and must) change the "requirements" for time/energy, as the time/energy available for the project is your core limitation that you should treat (in the short run) as out of your control, especially as it concerns the time/energy of others before funding. From the classic project management tradeoffs, this is a "limited budget - adjust scope to match" scenario, not "fixed scope - let's see what the budget will be" one.