The EU single market and customs union are all about protectionism. They do not like to compete. The SM and CU are the reasons there is starvation in Africa/Middle east. Highly subsidised EU farmers could not compete with world trade prices. Highly subsidised French industry could not compete with world trade price or practice. They have only one defence. Keep world trade out.
If the UK had reverted to WTO rules, the UK consumer would have had an immediate price cut on most consumer goods. The EU would have had a choice, either take a massive hit to profits and corporate earnings by matching the price, or lose its biggest customer and take a massive hit to profits and corporate earnings.
To the EU, free trade is kryptonite, it is the subject that shall not be named, it's anathema.
No deal was a real and potent existential threat to EU philosophy.
I would have preferred no deal. It would end crony corporatism, a smidgen of capitalism with the most efficient distribution of resources would have been a tonic.