minecraft resource pack previews are misleading for users looking for a resource pack that do not have shaders installed. most previews look nice because they have shaders, but even minecraft default with shaders looks nice. the non shader user viewing the pack will subconsciously like the preview because of the shaders that are shown with the resource pack in the preview.
Well thats kind of the owner problem, I myself have screenshots showing the pack without shaders, and have some with shaders, so whaever you position is, you can see what it looks like.
Now keep in mind that texture pack owners will always use the biggest resolution available, so if the pack comes in x256, it will be used for screen shots, because well the pack is ussualy made to look best at hier resolutions, the smaller ones are there to help people who cant tun the bigger ones.
Theres nothing you can do about it sadly, but hey downloading 100MB to try a pack out aint so much.
Its called marketing. Fast food chains often brush up their burgers for a photo shoot that will be used in their ads. The burger may look big and appetizing in the advertisements, but they are small and unattractive in reality. Same goes for resource packs.
I actually prefer for the author of the resource packs to include both screenshots with and without shaders. And since I've only included 2 photos with shaders on my resource pack, I should do something about it.