Sun Angel hasn't signed on since 2016. If you don't mind someone else answerimg.
Alfalfa is widely available, fairly affordable, and has a better nutritional profile than all the other such options with the exception of complete chicken feeds (like pellets or crumbles.) And is enjoyed by most chickens.
I would add, it is about as close to "fresh food" as one can find in stored feed. I think there is value in fresh food that does not show up on the labels of food for people or animals. Things like antioxidents, phytochemicals, endophytes...
Disclosure: I haven't fed alfalfa to my chickens yet. I've fed a lot of it to cattle and other species and read quite a lot about the details of feeding it to chickens. However, we generate too much good fresh food from our kitchen and/or from our land to want to buy alfalfa too. The more fresh food I offer, the more important it is that what I offer is in combinations that allow them to get enough of each nutrient without getting too much of any of the nutrients or anti-nutrients.