Hi Mintie and welcome to the Community.
A holiday for your daughter sounds amazing but as she is experiencing getting affordable Travel Insurance can be a challenge and at times the price quoted could be higher than the cost of the holiday and some people do find that no Insurance is being offered.
I personally had to be 2 years in remission and not on any cancer related treatments for my type of incurable blood cancer before I could get any price (and that was with my consultant saying I was clear to fly a good 18 months before) but these decisions are often all down to cancer types, treatment, prognosis and the resulting risk factor put on by the insurers risk assessment - I am very open to serious infections due to a reduced immune system so this was seen as a high risk factor for insurers.
Comparison sites tend to use the same algorithms so getting a clear price can be difficult and it’s not that unusual to be turned down or told to call the insurers.
It is actually more effective to call a few insurers directly as this means that you have answered all the questions correctly and in doing this a suitable policy can be offered and price given.
Full disclosure is always advisable as in the event of you developing any issues this can reduce the risk of an insurance companies wriggling out from paying out.
As a Macmillan Volunteer I can’t be perceived to recommend any companies but do have a look through the link below…….
……… that is an ongoing discussion thread (do go to the bottom of the thread and hit the >> to take you to the newest posts) in our Travel insurance group as it's where the majority of recommendations from other forum members are.
You can also look through all the individual members of posts in the group as well.
I do hope she can get cover.
Yes if I use a comparison site I can’t get a quote but I do when I call direct 
