I've looked into this quite a bit and found collections in the National Archives and various websites, but they are small and highlight the best photos. Do you really need to travel to archives or order collections to see photos? Especially now with such a internet based society, it seems important to have as many of these pictures online and easily accessible so that people don't forget.
Ok, we're an "internet based society" now, but we weren't then. How does all that physical media from before the Internet get onto the Internet? Somebody has to scan them, and catalog them, and store them. And then who pays to keep it there?
High quality digital archiving is slow, expensive, and labor intensive. Most museums have an enormous backlog of items to examine and display, and very little money to do it.
Before you can create a digital archive, you need to go through the normal steps for archiving media.
- Get your hands on the media.
- Figure out how to look at it without damaging it.
- Figure out what it's of.
- Figure out if it's of historical significance.
- Figure out who owns it.
- Get permission to display it online.
Many of those photographs are still under copyright, and finding out who owns it can be very hard. Displaying a photograph in a museum is one thing. Displaying it on the Internet where it can be easily copied is another.
Much of it will be in poor condition, or only as negatives. Simply handling and unrolling a roll of brittle old film might damage it.
Then you can go about digitizing them. This isn't a matter of going down to Best Buy and getting a consumer flatbed scanner. Doing this efficiently, at archival quality, and without damaging the photographs requires special equipment and training.
Then there's restoration. Most film from the 40s will have degraded. Again, this isn't a matter of hitting the "Auto Color Correct" button on your image software. Sharpening and color correction must be done in a way that retains historical accuracy.
Then you need to store them. Again, you don't just buy a consumer hard drive or dump it into cloud storage. You need reliable, future-proof, long-term storage with solid backups.
Then you need to tag, describe, and organize them into useful collections, this is labor intensive and must be done by historians.
Then you need a site they can be downloaded from. It needs to be cheap, ad-free, and of academic quality (ie. not YouTube). If you've looked at most museums, they don't have very good websites. Hosting them locally will mean big bandwidth bills.
Archive.org provides an archival service. They charge $3 (USD) to setup and $0.10 per image. You get non-destructive scanning, archival, OCR, and search. They also provide Archive-It. The Internet Archive works with the Open Content Alliance to "help build a permanent archive of multilingual digitized text and multimedia content".
And yes, they do have an archive of WWII photos and film.
Nowadays is still less expensive to store or backup in tape or other offline media than in hard drive. Since information stored in offline media can't be reached through internet that data has to be reached on site. Besides, if you share information on the internet you also need to pay the internet connectivity and the digitalization process, which is still too expensive.
Summary, you need to pay more to get that data online. Because the incentive is to keep the files offline.