A very interesting question.
Kale, cabbage and indeed loads of other things like cauliflower, Brussels, swede, kohl rabi etc are all cultivars of the Brassica oleracea plant species. A remarkable plant, it has the interesting property that it can, with no great difficulty, be adapted to store up energy and food reserves in many different parts of the plant. Depending on where the food reserves are stored - leaves, roots, base of stem, terminal bud, lateral bud, and inflorescence results in very different crops.
Getting back to your question, I suspect one key issue is cooking method. Also some sorts of cabbage may stand for a long time during which the flavour can get a bit coarse. If boiling cabbage, we usually chop a small amount of onion into the water. That seems to sweeten the aroma. Variety could be an issue too. Some modern ones have also been selected to have a less intense and sweeter flavour.
I wonder if other members would suggest varieties that they consider best for flavour. I like kale but a few seasons ago grew a variety that tasted dreadful.