That's because chickens are omnivores... they require animal protein.
I raise BSF in the summer and freeze them so that the chickens have animal protein in the winter.
I want to start my own colony of BSFL but having issues with attracting a wild colony I know there out here there just being difficult, anyway you could ship me some BSFL so I can get a colony started? If so PM me and ill give you my shipping info.
You'll probably have to wait until next spring to really be able to raise them. It's just too late in the year.
The trick to attracting them in the spring is to put bait buckets around your property. I use 5 gal buckets with about an inch of fermenting cracked corn in the bottom. Rig some wire across the bucket so that you have corrugated cardboard hanging over the fermenting corn. Then place the lid on but don't seal it, just crack it slightly.
I wouldn't bother even looking in it the first few weeks because it'll be nasty and attract the wrong kinds of bugs at first. But after the temps are consistently above 60 at night (70 is even better) you can pull out the cardboard and should find clutches of tiny yellow eggs. Several of those clutches set on top of the fermenting food in your bin and you'll have a population explosion within about 3 weeks.