Ultimately it's because giffgaff have to make a profit.
Most people don't make that many voice calls, and probably don't send many text messages, so the recent(ish) change to unlimited texts and minutes didn't really cost giffgaff anything.
However, people do use data, so it's in giffgaff's interests to try and charge people a little more to increase their data allowance.
When you remember that giffgaff make on average £11 profit per customer per year, it becomes clear that increasing the allowance for the £6 goodybag isn't sensible if it's going to lead to significant numbers of people on the £8 goodybag trading down.
