I've observed this on just about every TFS flight I've taken in the past year, with 10+ rows full in CE on every flight and ET 40% full at most. { Only exception to this was the flight home on Friday where ET was full, but it was the last weekend of the school holidays. }
Would suggest the factors in play on the holiday buses currently causing this are....
1. The PUGs are £20 - £30 cheaper than pre covid.
2. Because flights are not reaching anywhere near capacity, the PUGs are being offered on nearly all flights at some point.
3. Couples are upgrading so they don't have anyone next to them.
4. Because of the additional costs of tests, a higher ratio of the more affluent are travelling ATM, which coupled with the reduction in flights means more natural demand for CE vs ET.