Hello,
I'm a Certified Diabetes Educator. The reason that ReliOn-brand insulin (available only at Walmart) is not accepted by insurances is that ReliOn insulin can be purchased over-the-counter. Since it available over-the-counter, insurance companies won't cover it.
For people in the Medicare donut-hole or for people who's insurance companies still make the cost of insulin unaffordable, ReliOn brand insulin is an option. Each vial costs ~$25. They are the older type of insulins, but better than none.
The insulins available via ReliOn brand are: (I'm not sure if it's Humulin or Novolin)
NPH (Humulin/Novolin N) -- intermediate acting insulin
Regular (Humulin/Novolin R) -- short-acting insulin
Humulin/Novolin 70/30 -- mixed insulin
I hope this info is helpful.