I think there's a lack of trust in the ECJ. Obviously it's also a political thing - the tabloids don't like policies they approve of being over-ruled by courts from their own country, let alone nasty foreign ones.
But it's also a trust issue. For example at Maastricht the UK negotiated an opt-out from the Social Chapter. Later signed up to by Labour. But I can remember the ECJ ruling on UK holiday pay entitlement that the social chapter should apply to the UK in that area because of "health and safety". It's always been seen as a federalist court, in that it often rules in the way that gives the most power to the EU in cases that effect where power should lie.
After all, the treaties were pretty clear that Article 50 shouldn't be reversible without a unanimous vote of all members once activated. But the ECJ ruled that this was not the case, against both the legal opinions of the Commission and the British governement and English courts. The High Court ruling on Gina Miller's first case was that A50 had to be activated by Parliament not the government because it was irreversible.
So the UK government's attituded is that the ECJ is an EU institution and will always rule in its favour in disputes between us and them. So they can't be trusted to mark their own homework. Which is why most international trade agreements have arbitration panels that are independent of both sides.