I think it's because Ginette has no interest. If Ginette had interest, Elle could just tag along on a free trip and try to wheedle her mum into upgrades e.g. better hotels, better restaurants
but if Ginette has no interest, it would be difficult for Elle to convince her mum to go on these trips + for her to pay for them.
The problem is that Ginette won't naturally want to spend her money on a luxurious vacation (see Tahiti cruise trip that Elle made a big deal over because she paid). So it's not so easy for Elle to get to go on such vacations.
Honestly the real problem is that she has no friends. Everyone who studied in the UK went on trips all the time, there were extremely affordable flights or Eurostar to Paris / Belgium etc. And if you wanted to be bougie, there's no lack of wealthy students to travel with?! Many have family homes abroad or go on ski trips during the many school breaks?!
Elle is just in a very awkward position. She can't fit in with the rich kids (she can't fit in with wealthy adults either for that matter), she can't find regular friends to go on trips with, and she's attached to the hip with a mother who is perfectly content in a very small bubble - and hence so is she.
At the end of the day, I don't think Elle wants to travel if it's not 5*. So if nobody is offering to pay for it, she won't go. Whereas Hawaii is cheap for Elle, and once she goes over, it's also easy to wheedle her mum to splurge on a couple of meals and a present. and most importantly, she gets the "optics" and can brag about her mother's "property" in Hawaii to impress average Joes (clearly it didn't fool the most important JOE of all).