Lots of startups get stuck
before they get to Product-Market-Fit.

What would be the reasons?

Here are some, I see on a daily basis. They are related to issues with founders or market (in no particular order):

1) Founders haven't studied or trained on basic things of startups: idea validation, market validation, customer value-proposition, team-building, product building, basic finance, and total money required to do few iterations.
A: Work for a startup for a few years and learn.

2) Founders are solving a problem that they face in their daily life at home or work and start solving for themselves before checking whether there are others who care about the same problem. After building the product, they realize the market issue.
A: Idea validation failure

3) Too headstrong and think every potential customer can't imagine the value unless they experience the product. Hence, start building the product.
A: If you are deep and know you are at the same league as in Steve Jobs, this makes sense

4) Founders talk to few friends and colleagues and start building a product. After building the product, they realize there is no high demand for the product.
A: Spend month(s) on the problem and not on the solution.