Are you asking why Hikari food has not healed his tail? Hikari food won't help with health issues, water changes will.
I haven't used Hikari betta pellets for years so they may have changed the ingredients, but last time I had some it contained a lot of fillers and fish meal, which are not the best of ingredients.
I know that cherry barbs are one of the most non-aggressive barbs, but they may still see a betta's tail as something they just cannot resist.