Its top speed was about 30-40mph higher than its takeoff speed.....
And making the Ju52 have anything higher than 10 troops would only penalize the C47 pilots, since HT said "I can make it carry any number I want, but it'll still take a full plane to capture a field" [paraphrased].
Meaning, Ju52 carries 20 troops, maprooms now take 20 troops, and C47 pilots get shafted.
10 is chosen for gameplay consideration. It's not a historical number. Ju52 or any other major troop plane in this game would carry 10 as well, just because that's the way the game is played.