Why do light novels tend not to be animated directly?
I'm not an industry insider but it's not a deliberate matter of testing the market with manga before doing an anime. 

It's simply that an anime is a much larger investment for all parties involved with a much longer lead time. As a result, the chances are, by the time a LN has passed through the various investors and producers required to greenlight it it has already been taken up by a mangaka. In the typical case, it's just so much easier to get a manga deal done first. The popularity of the manga itself does not directly affects the anime, the popularity (in some cases just the quality) of the original LN is enough to make a project happen.