If this is indicative of the impact study DeExEU have done into 50+ sectors of the UK economy it's not hard to see why HMG don't want to release it.
Very, very few people now think that leaving the EU is a good thing fiscally for the country in the short or medium term, even moving to EEA/EFTA terms, even worse for WTO, obviously long term predictions are very fluid, and the economy is a massive influence on elections. A government pursuing policies that it, itself, thinks will weaken the economy is not a big vote winner.