They were "the" iconic pistol for the German armed forces for almost forty years, and a lot of WW2 vets had already been raised by men who'd brought plenty of them home in the first World War.

They've got a fairly uncommon toggle-lock design (not a lot of other pistols utilize that style of action, the Borchardt comes to mind, and it's really just a predecessor to the Luger), they're well-made, and there were a dizzying variety of variants and sub-models.

Also, realize that in WW2, not every German pistol issued was a Luger, which made them a bit harder to come across. The Germans never had enough small arms, and pressed into service whatever stocks were captured when they overran a country. /u/r_shackleford did an excellent overview of a large variety of German-issued pistols in WW2. It can be found here.