One simple reason; your risk situation changes. Surprisingly so. I have found many situations where what was covered and covered reasonably well, is not only months later.
For example, a new client I was talking to had a cover that was only a few months old. The situation had changed quite quickly following a relationship break up, quite a bit of debt had been taken on as a result. The life cover taken was no longer sufficient to cover this and the repayment insurance they had, now has a significant shortfall.
Another client after reviewing their situation, a month later comes back. Oh yes, by the way, we have bought an investment property and effectively doubled our mortgage. Quite a change in risk there too.