Why do canned sardines have no protein scum?
Hard to answer concisely, because there are many variations in processing before the Sardine hits the canning retort (a sterilization pressure cooker). Recall that there are sardines "in oil" "in spring water," even in "hot sauce." Here is a video example.

Sardines are canned under both pressure and temperature in the canning retort. I believe the pressure factor has more impact in supressing what you call scum ** though temp prolly plays a factor as well.

Home brew example: I rarely find scum like formations in my pressure cooker. I do find congealed fat on cool down.. but congealed protein.. not so much. 

** Not sure if the scum you are thinking of is congealed fat, or protein.