Why do light novels tend not to be animated directly? Based on the answer to this question, it seems that light novels that are turned into anime are generally made into manga first (eg Haruhi Suzumiya). There are some exceptions, such as Love, Chunibyo &amp; Other Delusions where an ONA was released of it before a full anime and Toaru Hikūshi e no Koiuta where the manga began a month after the anime. A lot of other LNs are never turned into anime at all, including ones that exist in otherwise existing anime canons (eg the light novels set in the Naruto canon). 

Are the ones that are made into a manga first done so to determine if they'll be financially successful in visual form, and that's why they're not made directly into anime? Why then would ones that are part of popular series (Naruto, Death Note, etc.) not be made into anime? Is there a reason why we so rarely if ever see light novels turned directly into anime?
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.