In case images are requested of a slow source, aborting image requests that are already in progress will cause the server to take forever for new source image requests because it's still processing the aborted image requests.
Thus, aborting ongoing image request might cause more harm than good, since the server will basically become "unresponsive" for new images requests of this source.
By only aborting pending requests, that have not been sent to the server yet, new image requests will only be handled once the active requests are done, preventing the server from getting spammed with image requests that potentially are just getting aborted immediately again (e.g. due to scrolling quickly in the source browse page), but still will be handled by the server, blocking new requests
In the reader the preloaded page requests were triggered before the actual page got rendered and thus, the image request of the rendered page had to wait for all the preloaded images to finish
schroda