Thats cause they still have a bad reputation from old times. They are niche units but tbh they actualy are a lot less niche then most niche units. A good samurai can easily take a heavys place in your army in most of the cases or in those where heavy def is needed as second HA. I got shizuka as my thrid shrine plat so i raised her and AWed her (also cause of her synergie with saki) and i did take her in 9 out of 10 times over my AW bernice.