Also, from their latest review by S&P, note the description as "very strong capital and liquidity":
Summary: National Western Life Insurance Co.
The insurer financial strength rating on National Western Life Insurance Co. (NWLIC) is based on its very strong capital and liquidity profile, strong operating performance, and strong competitive position in the international life insurance market. ...
I think its mainly the thinly traded nature of the stock as well... Had the good fortune of speaking with a major holder of the name and he concurred. When funds have to provide nightly or een monthly liquidity it makes things a bit more complicated. I know from personal experience, when you work for the wrong boss with the wrong investment focus, you can look very foolish in a stock like NWLI no matter how cheap.
Thats why its such a great stock for those with an ultra-long term investment horizon..