About "So... for all above listed campaigns... there are no incentives... why would those contributors do that ???"
Perhaps it's due because people read "2 prizes : 1st prize : USD50 ; 2nd prize: USD25" (for the 377 campaign ) and forget the closing date over their excitement at this kind of financial gratification.

Info: the issue is still there. I tried to upload a new version of File:Mühle_Oberding_01.png and I'm always getting this:

FYI, my test subject was this image: https://commons.wikimedia.org/wiki/File:Englischer_Garten_Meiningen,_Gruftkapelle_-_2020-04-29_HBP.jpg
The actual upload, after the failed test with Special:Upload, was done with the Upload wizard. But I still prefer the Special:Upload form, as it gives me more leverage on the {{Information}} template contents.

I just tested uploading a photo of 17,6MB, and the effect (getting "Service Temporarily Unavailable
Our servers are currently under maintenance or experiencing a technical problem. Please try again in a few minutes.") is still there.

Gunnex incidentally linked the foregoing Commons Village Pump discussion about the very same subject just above and more or less suggested that the following suggestions may be copied here from there. Fellow contributor Magog the Ogre suggested some kind of communicating interface. I worked on his ideas and hence want to suggest the following changes / improvements to be shown in the cross-wiki upload dialogue: