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Harriette's first point of "a suitable pattern of rainfall for collection" is the most common reason for rainwater harvesting to be deemed economically infeasible in projects I have worked on.

The issue is that you have a base-load of water demand for potable purposes. This is relatively constant, hence you need a solution that can meet this demand all year round. If rainfall is seasonal, then you either need a back-up source when there is no rain, or massive storage to see you through the dry spells. If you are storing, then you also need to keep the water from going stagnant. If you have a back-up source then it is often more economical to use that all year rather than providing the treatment plant to make rainwater potable.

It is easier to use rainfall for irrigation than treating for potable use, but again, in areas of the world with highly seasonal rainfall, you end up with irrigation water at the time when fields are waterlogged from the rains anyway.

Conversely, if you have regular rainfall, interspersed with shorter dry periods, rainwater harvesting for irrigation can be a highly effective solution. If you have extremely regular, year-round rain, then treatment of rainwater for potable use can become economically feasible too.