Historically, both domestic stock and wildlife had to produce and reproduce for mankind given only the resources available upon the landscape. Recently, modernization of agriculture placed new plants upon the landscape and soil amendments in an effort for man to better provide for the productivity of the animal. During the course of that transition, there were steep learning curves for both the animal and for mankind. Those learning curves have become what is called 'nutritional wisdom'....moreso for the animal than for the man...and often at the animal's expense!
Wild brassica and related plants are common to compacted soil and/or among overgrazed pasture plants....never seen them in a forest setting and rare in good rangeland. In some areas, it is likely deer haven't had the opportunity to use these plants in their diet and have no 'nutritional wisdom' of using brassica. So naturally when you plant a crop type brassica it may take years before deer develop the wisdom needed to include that plant in the diet....all depends on past related wild plants on offer. All plants contain both toxins and nutrients to whit the animal must learn to deal with!
Deer will figure out how to use the many plants you have on offer long before you will figure out how, when and why they use them. Such things are easier to figure out in lean times but less so in abundant times. Brassica (in moderation) do a good job of improving soil physical properties and enhancing soil nutrient recycling rate. That is far more important for health and growth of future plants or companion plants and long term soil quality, than if the animal uses the plant at all. WHEN YOU WANT the animal to use the plant may not be the time when the animal want's to use the plant. Having grown brassica in mixtures practically year around.....I can tell you that deer will use them at practically all stages of growth in varying degrees, but when they use them varies tremendously from year to year and I can't tell you why or when they will use them! Plants contain primary, secondary and tertiary chemical compounds which constitute what we call 'nutrients' and 'toxins'.....the animal is far superior in figuring out which of these compounds are needed in their diet and remedies for compounds from other plants....moreso than the modern science of animal selection and preference can figure out. If you don't believe that deer have evolved to deal with plant toxins, then the next time you open one up take a hard look at the liver in relation to body size....it is a huge % of body weight....and the liver is a detoxifying organ and an organ of metabolizing nutrients...feedback from the liver and other organs drives plant selection through chemical and neural signals received by the gray matter of the brain!
Related example to your sudden brassica use....I was sitting a stand recently overlooking a small mixed warm and cool season plot. A doe and fawn eased through and the doe was actively selecting the top 1/3rd of sorghums regrowth (could have been millet or EW or WGS or SS).....I thought that quite odd and texted deerpatch the observation. Earlier in the week, I eased through the destination area and it was swarming with bees and wasps....I looked around for flowers and there were none! The sorghums were exuding a sugary sap from the leaves and that is what multitudes of insects were feeding upon. Recent weather patterns may have increased the sugar content of sorghums leaf and that is why the deer were using them.....who knows? The point is that that a plant is rich in a compound at a time when use of that compound is favorable for deer and insect use.
When choosing seeds to plant in a food plot, opt for 1) seeds of plants which are needed to address the soil limitations you are dealing with, 2) seeds which increase biodiversity, and 3) seeds of plants which are know to have broad deer use (even if you don't see some of those plants used heavily). Just enjoy watching the animals use what you have provided and take note of novel observations! Remember one thing, nutritional wisdom of the animal is a book with very long history!