In the same way you cannot buy fire insurance while your house is burning, most insurers will not cover you for IP infringement that you know about – or should have known about.
Underwriters will often do patent searches, litigation searches, and competitive analysis to uncover the risks of your IP – either the risk that you will need to enforce your IP or the risk that you might be sued by someone else. Some industries are more litigious than others, and so insurance costs more.
Some insurance policies may have a delay, such as 90 days, where they will not pay a claim for litigation. They make that up by extending the policy period for 90 days on the back end.
The pre-existing conditions limitation is an incentive for you to get insurance early – well before you run into trouble. Not only can you sleep better, you will have the resources there when you need it.