Besides the already mentioned reasons for not using sponsor content - the content on "most" reality based sites and a heck of a lot of others nowadays is not real pictures, they are video caps - and most of the LLs will not accept free sites built with vid caps (they are usually really poor quality)
I've also noticed lately that even the sponsors that do provide real pictures are way overcompressing the full size pictures while making top-notch thumbs to go with them - again a major reason why free sites get declined.
Most larger LLs get many submits a day - sometimes hundreds (some even thousands) and in those submits you can see a group of 10 free sites built using the same exact content (whatever the fad is today). Again - a reason to turn them down.
Most LLs won't accept sponsor content if they already have the same content listed - kinda sucks for a surfer to wade through the same pic set over and over.
There are also some LLs that wont list sponsor content for some other reasons - like they build their own free sites to list on their own LLs using the same sponsors content (why would the want competition?) and last - some sposnors are not welcome at some LLs due to problems we have had in the past with certain sponsors like cheating, skimming, copyright infringement and the list goes on.
As you can see there are numerous reasons that a sponsors content would not be listed - and then of course there is the group of LL owners that think (and I happen to be partial to this side of things) that if you cant shell out $6 for a set of content you aren't really serious about the business - for someone just starting out, a daily expenditure of $6 towards making your business grow is nothing compared to what it costs to start a real business
Of course if this is just a hobby - then we dont want the submits anyway - 95% of them have to be removed in the first 6 months after listing, another 4% 1 year after listing and the 1% that stay listed are the ones that spent the $6 for the set of pics and are serious about making this their livlihood - sorry to be so blank about it but we just went through (and have already spent a few hundred hours over the last three weeks) removing over 9,000 links to hobbyists and the people that submitted sponsor content.
Last - most LL owners list in their rules whether ro not they accept sponnor content - should be pretty easy to figure out which LLs might list the stuff