Economies of scale, the more passengers you have the lower your costs, as Bristol has about 5 times as many passengers as Cardiff they'll be able to offer lower fares from there, which is what the low cost operators trade on.
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Here are the passenger numbers for Cardiff airport - with the Welsh Government stepping in in 2013 wasn't it? you can see why.
If they can keep the numbers increasing then the low cost carriers would return as soon as the costs come down.
However the whole industry faces a very tough time over the next few years as following the covid crisis the whole industry will have changed.
I think they should focus on flights to the Americas. Have a US immigration in the airport like they have as Dublin airport and include a fast train link to London you could coax some traffic away from the london area -
and as the planes won't have to fly as far (or queue as long) they would save on fuel as well.