Travel insurance policies are just like any other insurance contracts - they have limitations and exclusions.
Travel insurance plans don't cover everything. There are limitations and exclusions that affect coverage. When reading a travel insurance policy you should always read and understand:
The single greatest exclusion that you have control over is for "pre-existing medical conditions". It's estimated that over 25% of all travel insurance claims can be traced to a pre-existing medical condition, but it's the one exclusion that you can control by finding a plan that will "waive" the exclusion in exchange for you meeting some coverage requirements.